Support for nesting channels aka subchannels

[ara4n]

Slack users like how different slacks group together sets of channels, and miss this grouping in Riot. As well as 'team directories' we should have a way to auto tag or group rooms together. Custom tags may actually be sufficient for this, although the most common request is for nesting.

[knewter]

My $0.02 - it's best if they're namespaced (even if this is just at the UI level) so if I join the #elm riot channel, I could have channels like #general, #random, within it, rather than having a million channels at the top like #elm-general, #elm-random, etc.

[ara4n]

This is almost the same as #2454

[richvdh]

This is almost the same as #2454

Yes. What's the difference?

[ara4n]

I thought #2454 was asking for the thing where if you join one room it autojoins you to various 'subrooms'. I was wrong; will add a different bug for that.