Implement Google Oauth2 login + Account Linking

[stadust]

Allow linking google accounts to pointercrate accounts, to allow signing in through google. This then eliminates the option to sign in with pointercrate password, but also means you can no longer lock yourself out of your pointercrate account as easily (unless you also lock yourself out of your google account, but really, what are you doing?).

This is a very bare-bones implementation:

• No registration with google possible yet (you must register for a legacy account, and then link)
• No changing of linked google account

Essentially, we're implementing Phase 1 and 2 of #127, leaving Phase 3 and especially 4 for later work.

License Acceptance

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[codecov]

Codecov Report

Attention: Patch coverage is 16.37168% with 189 lines in your changes missing coverage. Please review.

Project coverage is 30.17%. Comparing base (4da49a1) to head (6ee7058).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
pointercrate-user-api/src/oauth.rs	0.00%	46 Missing ⚠️
pointercrate-user-api/src/pages.rs	6.12%	46 Missing ⚠️
pointercrate-user/src/auth/oauth/post.rs	0.00%	25 Missing ⚠️
pointercrate-user/src/auth/oauth/get.rs	0.00%	15 Missing ⚠️
pointercrate-core-pages/src/head.rs	0.00%	13 Missing ⚠️
pointercrate-user/src/auth/oauth/patch.rs	0.00%	9 Missing ⚠️
pointercrate-user/src/auth/patch.rs	18.18%	9 Missing ⚠️
pointercrate-user-pages/src/login.rs	0.00%	8 Missing ⚠️
pointercrate-user-pages/src/account/profile.rs	0.00%	6 Missing ⚠️
pointercrate-user/src/auth/oauth/mod.rs	33.33%	6 Missing ⚠️
... and 3 more

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #229      +/-   ##
==========================================
- Coverage   30.36%   30.17%   -0.20%     
==========================================
  Files         113      118       +5     
  Lines        8012     8126     +114     
==========================================
+ Hits         2433     2452      +19     
- Misses       5579     5674      +95     

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