Add an admin API `GET /_synapse/admin/v1/users/<user_id>/pushers` like https://matrix.org/docs/spec/client_server/latest#get-matrix-client-r0-pushers

Add admin API `GET /_synapse/admin/v1/users/<user_id>/media` to get information of users' uploaded files.

Add a new admin API `GET /_synapse/admin/v1/users/<user_id>/joined_rooms` to
list all rooms where a user is a member.

Fixes https://github.com/matrix-org/synapse/issues/8185

Some fixes to wording I noticed after merging #7377.

* Search in columns 'name' and 'displayname' in the admin users endpoint

Signed-off-by: Manuel Stahl <manuel.stahl@awesome-technologies.de>

* Clarify how to authenticate
* path params are not the same thing as query params
* Fix documentation for `/_synapse/admin/v2/users/<user_id>`

- Admin is able to
  - change displaynames
  - delete devices
  - list devices
  - get device informations

Fixes #7330

Signed-off-by: Manuel Stahl <manuel.stahl@awesome-technologies.de>

Update documentation to reflect the correct format of user_id (fully qualified).

Signed-off-by: Manuel Stahl <manuel.stahl@awesome-technologies.de>

Signed-off-by: Olivier Wilkinson (reivilibre) <olivier@librepush.net>

Admin API: Set adminship of a user

I haven't (yet) documented all of the user-list APIs introduced in
https://github.com/matrix-org/synapse/pull/1784 because the API shape seems
very odd, given the functionality.