API to add server admins (SYN-82)

[matrixbot]

(Imported from https://matrix.org/jira/browse/SYN-82)

(Reported by @erikjohnston)

[matrixbot]

Jira watchers: @erikjohnston

[matrixbot]

Links exported from Jira:

relates to SYN-64

[rxl881]

As I can't see the JIRA item that this was imported from, I'll clarify what I would like this to cover (for my use case).

It would be great to have an admin API that would allow an existing server admin to set other existing server users as admin (as well as the ability to remove that privilege).

In order to make this useful we would also need one of the existing APIs, or a new API return whether the admin flag is set for existing users.

@neilisfragile -- Is it possible to get this prioritised soon?

[richvdh]

this seems to have been done by #5878