login: Add auto-discovery of homeserver
- Split the greeter and the login screens, and improve the look according to the mockup in #738 (closed).
- Put non-icon SVG assets in a proper folder.
- Add auto-discovery following the spec.
- Keep the possibility to enter the homeserver manually if auto-discovery is unwanted.
- Validate the homeserver prior to password login.
- Fix the warning.
I also wanted to check that we support the login methods of the homeserver, but there's a bug in ruma (#796) that makes the deserialization of the response of the corresponding endpoint fail. So we'll have to implement it later.
Fixes #786 (closed), although we'll need to go back to using
AdwStatusPage when it's fixed.