The purpose of this PR is to prevent the synapse admin user from deleting himself.

It's very common for first time users to accidentally delete their own admin user and then find themselves in a situation which is challenging to recover from.

For this to work, a new React component UserPreventSelfDelete is introduced, which accepts a single prop of ownUserIsSelected. If true, it means that the DELETE for this user should be forbidden.

It works on:

• bulk users' selection - if own user is selected, bulk delete will show an alert and prevent the action
• delete user media button - inside the User Edit page
• delete button - on the bottom of the User Edit page
• the deactivate checkbox is marked as disabled on the User Edit page