user_ids aren't escaped when generating urls [sending invites, joining rooms, etc] for federation use.

[walle303]

As / is an allowed character in user ids, they need to be escaped when generating urls, because right now if you have a / in your username and try to join a room or send an invite over federation, synapse generates urls like .../@user/id/with/forward/slashes/in/it:domain.
These, for obvious reasons, do not work. Instead they should be properly escaped, so .../%40user%id%2Fwith%2Fforward%2Fslashes%2Fin%2Fit%3Adomain would be used instead, or, at least escape localparts, so .../@user%2Fid%2Fwith%2Fforward%2Fslashes%2Fin%2Fit:domain instead.

[richvdh]

fixed by #3082