Enable registration api

[Rinary1]

Re-enables registration api with security fixes, summary:

1. Added Email Validation for Registration and Confirmation.
2. Added Password minimal and maximum length(from 6 to 100, same as on page)
3. Added username minimal and maximum length(from 3 to 32, added to registration page)
4. Added rate limits for registration and confirmation:
   • 2 per hour for confirmation.
   • 5 per 15 minutes for registration.
5. Added security notice when user tries to register account on already registered email.
6. Fixed issue when confirmation can be sent on already confirmed email.

Makes some "out of topic" changes so as not to undermine "general equality":

1. Added Rate Limits for Reset Password and Authenticate.
2. Removed commented out code(nitpick).
3. Added Message in Bed Request in Authenticate(nitpick).
4. Added Email Validation for Reset Password.
5. Added dummy user check in Unauthorized path so that the response times are roughly the same.
   (The same approach as with the Security Notice; its purpose is simply to make the response times for successful and unsuccessful requests roughly the same, because before, unsuccessful response was faster)

[juliangiebel]

We're not allowing API based user registration to prevent automated account creation. I also wouldn't send an email when someone tries to register with an email that and I wouldn't send an email when someone tries to register with an email that already exists.

Fixing the re-confirmation bug is, as well as the confirmation rate limit and the dummy account check.

[juliangiebel]

Remove the registration api, the associated rate limit and the SendAccountExistsEmail email.
The rest looks fine. Sorry that you'll have to remove the bulk of your PR but registrations through the API aren't something we want to allow currently