Capabilities API

[hughns]

There a few places where the config options or policy set by the MAS administrator will impact the availability of some features in the UI.

For example: #1663 and #1813

As such we would like to expose the availability of some features in a GraphQL query.

Notes on the types of things that would need to be expressed:

• whether password auth is on or off
• whether a user can change their password
• whether a user can change their display name
• 3PIDs
• Avatar

[sandhose]

This was done in:

• Allow restricting some self-service capabilities #2562
• Add Self-service Password Change #2863
• graphql: Expose CAPTCHA config and whether password registration is enabled #3017