`admin` scope has been required by default for more than a year now
and all apps that use the API seems to request a proper scope by now.

Also removes incorrect claim that /api/v1/pleroma/chats supports
pagination and deprecates it.

Closes #2140

Helps prevent accidental button mashes from submitting incomplete posts

Related-to: #2496

in /api/v1/mutes and /api/v1/accounts/:id endpoints

to the instance adminAPI endpoint

to the user statuses adminAPI endpoint