Replace 'Choose a displayname' dialogue with a dialogue to choose a MXID

[lampholder]

The details of the new guest experience for Riot are on the project plan: element-hq/riot-meta#59

Instead of prompting unregistered users to pick a display name (for their automatically-generated @guest_123456:matrix.org mxid), instead we will invite users to pick a new mxid (for now throwing away their generated guest ID, which is fine because it won't have been allowed to say anything).

This dialogue should handle 'username in use' gracefully, ideally checking username availability in real time.

[lampholder]

Some UX suggestions:

Invitation to choose a username:

Appearance if chosen username is available:

Appearance if chosen username is unavailable:

Outstanding things to consider:

• When we check availability (with every keypress? After n seconds?)
• How we communicate permitted/otherwise characters (on keypress? Does it need to go into the prose?)

[lukebarnard1]

This will require Riot to be able to generate user passwords: #3600

[lampholder]

@AmandineLP @ara4n - what are your thoughts on the proposed wording/UX?

[ara4n]

So, i think this was a bit too wordy and scary, and I don't think we should be exposing MXIDs so aggressively to new users. How about:

---

To get started, please pick a username!

 _________________________

This will be your account name on the $foo homeserver, or you can pick a different server.

---

After picking a username, we'd obviously do whatever captcha dance is needed (if configured).

• We show this whenever the user is moving from read-only (i.e. 'guest') to read-write mode. In practice:
  • If they followed permalink of some kind, it's the point that they hit 'join' to actually join the room - i.e. the point we currently say "Please pick a display name" for Guests
  • If they just entered riot.im/app itself, the app will immediately show this prompt (over a 'blank' UI), and having created the account, the app will prod RiotBot to send an invite to the new user, which the app will autoaccept, and so the first screen the user sees will be RiotBot.
  • Alternatively, if the Riot deployment doesn't want to use RiotBot, then they should probably dump the user into RoomDirectory by default or a welcome page (re-add welcome_page option so that folks running their own riots without RTS can have a welcome page #3806, or the RTS welcome page).

For RTS:

• The user is dumped into the RTS as a guest (i.e. read-only mode). The LeftPanel is empty.
• If they try to join a room, then they get bumped to the full login flow - which prompts them for mxid, an email, auths them to join the team, does captcha.
• Having logged them in, we return them to the RTS Welcome Page, autojoin the RTS-defined rooms, and prod RiotBot to invite the user. (Alternatively we could route them to RiotBot; unsure which).