Add servers to MobX.

Don't pass dispatcher in, just provide it globally.
Fix: Allow clicking through to profile from server sidebar.

Can now collapse server categories.
Client remembers collapse state, incl. advanced appearance settings.

Add rounded corners to sidebar.

Feature: Remember what channel was opened last.
Channels: ESC to focus message box / cancel editing.

Servers: Fix various issues relating to voice channels.

Modularise server header.