Fix user join/leave on Matrix

[remram44]

Currently, no list of users is obtained from Gitter: a user shows up in the Matrix room when he first speaks, and is never removed.

• Shouldn't invite/join users everytime they speak
• The full list of users should be obtained when the room is created, and virtual users created then
• We should react to the user join/leave events and add/remove virtual users then

[remram44]

37329cc stores the list of rooms a virtual user is in, so we don't have to invite & join everytime.