Partial implementation of MSC3824

[hughns]

Requirements as taken from https://github.com/matrix-org/matrix-spec-proposals/pull/3824:

• [x] ~~support the m.login.sso auth flow~~ already supported
• [x] where a delegated_oidc_compatibility value of true is present on an m.login.sso then only offer that auth flow to the user
• [x] RECOMMENDED: label SSO button as "Continue"
• [x] append action=login and action=register parameters to the SSO redirect URLs
• [ ] sign post and link users to manage their account at the OP web UI given by MSC2965

Type of change

• [x] Feature
• [ ] Bugfix
• [x] Technical
• [ ] Other :

Content

Motivation and context

Screenshots / GIFs

Tests

• Step 1
• Step 2
• Step ...

Tested devices

• [ ] Physical
• [x] Emulator
• OS version(s):

Checklist

• [ ] Changes has been tested on an Android device or Android emulator with API 21
• [ ] UI change has been tested on both light and dark themes
• [ ] Accessibility has been taken into account. See https://github.com/vector-im/element-android/blob/develop/CONTRIBUTING.md#accessibility
• [x] Pull request is based on the develop branch
• [x] Pull request includes a new file under ./changelog.d. See https://github.com/vector-im/element-android/blob/develop/CONTRIBUTING.md#changelog
• [ ] Pull request includes screenshots or videos if containing UI changes
• [ ] Pull request includes a sign off
• [ ] You've made a self review of your PR
• [ ] If you have modified the screen flow, or added new screens to the application, you have updated the test UiAllScreensSanityTest.allScreensTest()