[Feature] Redirect remote user to internal ID

Resolve remote user's host name and user name into internal ID, and redirect to that.

• /remote-users/:username@:hostname -> /users/:id
• /remote-users/@:username@:hostname -> /users/:id
• /remote-users/:hostname/:username -> /users/:id

Usecase

Some implementations of decentralized social network, such as Misskey, have no explicit remote follow feature on their standard web UI. In case of following from Pleroma (e. g. pleroma.example.com) to Misskey (e. g. exampleuser@misskey.io), first we can open the URL https://pleroma.example.com/remote-user/@exampleuser@misskey.io, redirected into https://pleroma.example.com/users/INTERNAL_ID, and we can push the follow button.