Requires https://github.com/matrix-org/matrix-js-sdk/pull/432 for availability checking.
Changes:
- Redesign the dialog to look more like https://github.com/vector-im/riot-web/issues/3604#issuecomment-299226875
- Attempt to fix wrong password being stored by generating one per SetMxIdDialog (there's no issue tracking this for now, I shall open one if it persists)
- Backwards compatible with servers that don't support register/availability - a spinner will appear the first time a username is checked because server support can only be determined after a request.
- Rate-limited by a 2s debounce
- General style improvements
ad2ed12980