Stop accepting nicknames or ids in /api/v1/accounts/:id

Although this is less of a problem on instances with flake ids, older instances have cases where there is both a user with nickname being equal to the requested string and an id being equal to the requested string, resulting in a wrong account being displayed at the url (For example you would expect https://kawen.space/314 to display @314@kawen.space, but it displays @TheKinrar@mastodon.xyz instead). I propose fixing this by stopping to accept nicknames in /api/v1/accounts/:id and making a separate endpoint for requesting a user by their nickname /api/v1/accounts_by_nickname/:nickname.

Things needed:

• Implement /api/v1/accounts_by_nickname/:nickname
• Advertise https://instance.social/:nickname in the url property of actors, this will make the external links lead to it.
• Make the FE use only /api/v1/accounts_by_nickname/:nickname when resolving a user at /:nickname
• Make the FE use a different route for remote users that are addressed by id (i.e /accounts/) . The problem with changing /users/ to be id-only is that our AP ids are /users/:nickname, so visiting an AP id in-browser will not work, which is undesirable because some implementations don't honor url.
• Remove nickname support from /api/v1/accounts/:id

cc @lambadalambda @lanodan @hj @shpuld