Expose user blocking in the admin UI

[ThiefMaster]

There's User.is_blocked which is properly implemented in the backend but currently only settable via indico shell.

Ideally, the UI should:

• Let admins block/unblock users
• Show that a user account is blocked when viewing their profile and maybe also in the user search results
• Filter out blocked users when searching in non-admin areas (?)

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Since we don't have any amazing place to put admin-level stuff for specific users I would either:

• Put it on the "Accounts" tab of the user settings. That's the least weird place right now.
• Add a new "Admin" tab, but with just block/unblock it might look a bit naked

The actual blocking/unblocking would be done in a new RH (similar to what RHAPIBlockKey does for blocking/unblocking API keys), but the action should be logged as well (not sure why we don't do that for API keys)

For the information in the profile I would show a message like we currently do for the system user (base.html in the users module). The search results can simply use an icon.

[ThiefMaster]

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