Fix #13330

* Rate limit based on remote address IP, not on potential reverse proxy

* Limit rate of unauthenticated API requests further

* Rate-limit paging requests to one every 3 seconds

30 per 30 minutes, like media uploads

Deletions take a lot of resources to execute and cause a lot of
federation traffic, so it makes sense to decrease the number
someone can queue up through the API.

30 per 30 minutes

* Add REST API for creating an account

The method is available to apps with a token obtained via the client
credentials grant. It creates a user and account records, as well as
an access token for the app that initiated the request. The user is
unconfirmed, and an e-mail is sent as usual.

The method returns the access token, which the app should save for
later. The REST API is not available to users with unconfirmed
accounts, so the app must be smart to wait for the user to click a
link in their e-mail inbox.

The method is rate-limited by IP to 5 requests per 30 minutes.

* Redirect users back to app from confirmation if they were created with an app

* Add tests

* Return 403 on the method if registrations are not open

* Require agreement param to be true in the API when creating an account

* Code quality pass

* Typofix

* Update applications_controller_spec.rb

* Update applications_controller_spec.rb

The previous rate limit allowed to post media so fast that it is possible
to fill up the disk space even before an administrator notices. The new
rate limit is configured so that it takes 24 hours to eat 10 gigabytes:
10 * 1024 / 8 / (24 * 60 / 30) = 27 (which rounded to 30)

The period is set long so that it does not prevent from attaching several
media to one post, which would happen in a short period. For example,
if the period is 5 minutes, the rate limit would be:
10 * 1024 / 8 / (24 * 60 / 5) = 4

This long period allows to lift the limit up.

Previously each protected path had a separate rate limit. Now they're all in the same bucket, so people are more likely to hit one with register->login. Increasing to 25 per 5 minutes should be fine.

* Fix #668 - Rate limit by user instead of IP when API user is authenticated

* Fix code style issue

* Use request decorator provided by Doorkeeper

application/json only allows Unicode, so this prevents from wrong charset detection.

* Add rate limits for logins and sign-ups by IP (5 in 5 minutes)
Should be enough for normal attempts

* Add rate limit for forgotten password form as well

to the API

in the pool, adding a throttle of 60rpm per IP, adding mini profiler, adding
admin status to users