[Bf-interface] Official Blender UI IRC Channel Switch

[Julian Eisel]

Hey all!

So far, we've been using #blenderui as official Blender UI development
channel. After some back and forth with Freenode representatives, we
decided to switch this channel to follow the Freenode naming
convention. That would be #blender-ui. To make it clear:

!! The official Blender UI development channel now is #blender-ui. !!

So folks, please switch over :) We can technically add a redirect, but
I don't think that's needed.

This new channel is owned by us, so we can finally do things like
changing channel topic ;)
I've updated the needed Wiki links already.

Cheers,
- Julian -