Prevent accidental user overwrites

[aine-etke]

Synapse Admin API provides Create or modify API endpoint that allows overwriting existing users by default.

Sometimes, you may want such functionality, but there is a huge change that in the vast majority of cases that will lead to accidental/unexpected overwrites. This issue was already raised in Awesome-Technologies/synapse-admin#176, Awesome-Technologies/synapse-admin#467, and matrix-org/synapse#8165, but the API endpoint was not changed.

Luckily, a new endpoint was added to the Synapse Admin API, that can be used to prevent unexpected users overwrite: https://element-hq.github.io/synapse/latest/admin_api/user_admin_api.html#check-username-availability

Please, make Synapse Admin to call that endpoint before creating a new user, and if there is already existing user with this username, show a modal window with "Warning! This username is already taken. Are you certain that you intend to overwrite the existing user?" alert and options to continue (i.e., send the request), and cancel.

It would be amazing if that check could be added to the new user form near the Username field as well, so when you're entering the username, you will see something like "✔️ username is available" and "⚠️ username is already taken" messages near the input field