Fixing NullPointerException caused by having a mix of valid Groups and NULLs inside the result of getGroupsFromKeys

Openfire 4.2.0 will have the functionality that's now in the websocket plugin. As a result,
this plugin should not be used in Openfire 4.2.0. To facilitate this, we've defined a new concept
of 'priorToServerVersion' in OF-1353.

This commit defines that the plugin should only be used in Openfire instances prior to version 4.2.0.

websocket plugin has been broken since Openfire 4.1.4. This PR implements the missing code and fixes the issue

    @Override
   public ConnectionConfiguration getConfiguration()
   {
       // TODO Here we run into an issue with the ConnectionConfiguration introduced in Openfire 4:
       //      it is not extensible in the sense that unforeseen connection types can be added.
       //      For now, null is returned, as this object is likely to be unused (its lifecycle is
       //      not managed by a ConnectionListener instance).
       return null;
   }

* OF-1354 add option to CC to disable start a chat

* OF-1354 add option to CC to disable start a chat

* OF-1354 add option to CC to disable start a chat

* OF-1354 add option to CC to disable start a chat

* OF-1354 add option to CC to disable start a chat

* OF-1354 add option to CC to disable start a chat

* OF-1354 add option to CC to disable start a chat

* OF-1354 add option to CC to disable start a chat

* OF-1354 add option to CC to disable start a chat

* OF-1354 add option to CC to disable start a chat

* OF-1354 add option to CC to disable start a chat

* OF-1354 add option to CC to disable start a chat

* OF-1354 add option to CC to disable start a chat

* OF-1354 add option to CC to disable start a chat

* OF-1354 add option to CC to disable start a chat

* OF-1354 add option to CC to disable start a chat

This is a complex problem.  During setup, fetching xmppSettings from the session may fail due to an invalid session state maintained between the client browser and the server.  This is likely caused by a previous HTTPS session to the same server hostname setting a `JSESSIONID` that subsequently can not be overwritten by the current HTTP openfire-setup session.

This introduces i18n string `setup.invalid_session`

Usage of Mapped providers requires a lot of configuration, that should be documented. The documentation provided in this commit describes a common use case, in which admins are to be retrieved from the default providers, while regular users are obtained from an external system.

* OF-1359: Admin console changes should be reflected in changelog and version number.

* OF-1360: Update inVerse plugin to match upstream Converse 3.1.0 release.

Fixes OF-1308

OF-1350 MUC passwords

* OF-1343 added comments about changes in 4.1.5

* OF-1343 a note about new service behavior

The static web content handler is allowed to be null (when there's no static content to be served). In such case, the null-handler shouldn't be loaded in Jetty (which will throws nullpointers).

Various plugin-related improvements.

EditorConfig helps developers define and maintain consistent coding styles between different editors and IDEs.

* OF-546: UserImportExport plugin should have admin console when built with Maven.

* OF-1355: UserImportExport should not fail on missing optional config.

A new Openfire plugin, that bundles the third-party Candy web client.

A new Openfire plugin, that bundles the third party 'inVerse' web client.

'max' implies to specify a server version that still supports the plugin, while typically, one would like to specify the first version in which support was dropped. That version should thus be 'exclusive'.

XMPP defines two configuration fields for password-protecting a room:
- one that defines if the room is password protected.
- another one that defines the password.

Openfire, internally, only stores the password, if password-protection is required.

Some clients send only a password, without explicitly defining the password-protection boolean. In such cases, Openfire should be somewhat flexible, and process the configuration form as intended by the user.

This commit started with the intention of adding a maxServerVersion attribute for plugins, but resulted in a partial rewrite. Changes:
- Removed the License enumeration - now, a simple string is used.
- Split off the admin console page that shows the content of readme and changelog files to a dedicated page, instead of plugin-admin.jsp
- Better define the difference between the plugin (human readable) name ("Avatar Resizer") and its canonical name ("avatarresizer")
- The admin console now shows plugins that have been downloaded/installed, but cannot be loaded. Where possible, an update is suggested.
- Support for maxServerVersion - plugins can now define a version of Openfire in which they will no longer load.