Allow admin users to register new user account

[awesome-manuel]

Description:

Right now, there are two endpoints that can be used to register a new user:

• _matrix/client/r0/register
• _synapse/admin/v1/register

The normal endpoint either allows anyone to register a new account (setting enable_registration) or an app service or via shared secret. The admin endpoint only allows registration via shared secret.

My request is to allow any user flagged as admin to also register a new user, most probably via the admin endpoint. This could be limited via a setting, e.g. enable_admin_registration or changing enable_registration from bool to enum.

[neilisfragile]

Hi @awesome-manuel can you explain your use case, as you describe admins can already create accounts (with a shared secret) - I want to understand how that limits you.

[awesome-manuel]

@neilisfragile we're building a web admin UI for synapse, (see https://github.com/Awesome-Technologies/synapse-admin) and obviously we don't want to put the shared secret into the react application. For the admin UI it should be enough to login with admin account credentials.

[neilisfragile]

Right I see, sounds good, thanks for the PR #5742

[richvdh]

now done via #6495: thank you!