Add password reset

[freezy]

This adds an API to reset a user's password. It works like that:

1. The user requests a password reset for a given email
2. An email is sent with a link to reset
3. The link contains a token, with which the user can set a new password

Notes

• The API returns if the email is unknown (no "If we found your email, we've sent you one" message)
• If the email is registered with an OAuth account that doesn't have a password, the name of the OAuth provider is returned.
• The token is valid for 24 hours.
• There is no "forgot email" link. The email has to be known.

Todo

• Rate limit the reset and request endpoint

This PR closes #264.

[codecov]

Codecov Report

Merging #295 into master will increase coverage by 5.04%.
The diff coverage is 89.39%.

@@            Coverage Diff             @@
##           master     #295      +/-   ##
==========================================
+ Coverage   85.65%   90.69%   +5.04%     
==========================================
  Files         175      175              
  Lines        6572     6620      +48     
  Branches      530      535       +5     
==========================================
+ Hits         5629     6004     +375     
+ Misses        801      473     -328     
- Partials      142      143       +1

Impacted Files	Coverage Δ
src/app/common/slackbot.ts	100% <ø> (ø)	⬆️
src/app/releases/release.api.ts	96.91% <ø> (+6.17%)	⬆️
src/app/users/user.api.ts	80.85% <100%> (+31.2%)	⬆️
src/app/profile/profile.api.router.ts	100% <100%> (ø)	⬆️
src/app/common/api.ts	77.41% <100%> (+4.03%)	⬆️
src/app/media/medium.api.ts	100% <100%> (ø)	⬆️
src/app/comments/comment.api.ts	100% <100%> (ø)	⬆️
src/app/authentication/authentication.api.ts	98.36% <100%> (ø)	⬆️
src/app/users/user.schema.ts	85.98% <63.63%> (-0.48%)	⬇️
src/app/common/mailer.ts	82.9% <66.66%> (+3.77%)	⬆️
... and 35 more

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