Keep only one connection per server

[WIP] New rewritten push notification handler

Notifications below on Android below Nougat should only be grouped if there is more than one message

Merge branch 'fix/notifications' of github.com:RocketChat/Rocket.Chat.Android into fix/notifications

Remove dismissed notitications considering also the hostname to avoid unintentional removal when in multiserver

[NEW] Added support for Delete message #347

Set roomId on extras sent from the push message to the app if tap on single conversation, otherwise tapping on bundled (or grouped) conversation let the app open on the last selected roomId

Fix link to our Coding Style guidelines new location

Fix caching of Site_Url and Site_Name by no relying on current selected server anymore; add bundled notifications

notification.