Finishing off the first iteration on login UI
This makes the following changes:
- Improve CountryDropdown by allowing all countries to be displayed at once and using PNGs for performance (trading of quality - the pngs are scaled down from 32px to 25px)
- "I want to sign in with" dropdown to select login method
- MXID login field that suffixes HS domain (whether custom or matrix.org) and prefixes "@"
- Email field which is secretly the same as the username field but with a different placeholder
- No more login flickering when changing ServerConfig (!) fixes https://github.com/vector-im/riot-web/issues/1517
This implements most of the design in https://github.com/vector-im/riot-web/issues/3524 but neglects the phone number login:
This will be updated in another PR to implement desired things:
- Country code visible once a country has been selected (propbably but as a prefix to the phone number input box.
- Use square flags
- Move CountryDropdown above phone input and make it show the full country name when not expanded
- Auto-select country based on IP
