New: user details endpoint

GET /api/alpha/user/me - retrieves all the information about the current user, including all the things they’ve blocked, communities they have joined and communities they moderate.

I built this because I realised that many apps are making requests to /api/alpha/site for this purpose, which is unfortunate as that endpoint returns a lot more besides and can sometimes be slow to do so. This is intended to be a more lightweight alternative to doing a GET of /api/alpha/site so the information in /user/me is the same as the my_user part of /site.