Add /openui <name> slash command

Allows a custom UI to be opened within the client.
Merge pull request #2 from Mehuge/feature-chat-openui

Add /openui <name> slash command
Community banner and realm specific backgrounds

Add community banner and realm specific backgrounds
Revert "Community banner and realm specific backgrounds"

This reverts commit 336bec79cfa36ffbad361e9934887829feef6f3b.
Merge remote-tracking branch 'upstream/master'