Add support for admin endpoints

[dorianim]

Hey there,

it would be great to have support for the /v1/admin endpoints as they are very useful for automated moderation :)

[lmas]

Sort of fuzzily related, I'm missing support for /oauth/revoke too.

@mattn I'd be willing to work on this issue and provide a PR, if you are ok with it.

Edit: Hmm just noticed gotosocial doesn't support the /oauth/revoke endpoint yet, so my request should be put on hold.