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.