More refactors to prepare for oauth login

[stadust]

Changed my mind overnight about how to implement this. Now the type system also enforces that things like invalidate_access_tokens are correctly implemented for all auth types. Also hide the registration web form if the legacy_accounts features is disabled.

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[codecov[bot]]

Codecov Report

Attention: Patch coverage is 56.03865% with 91 lines in your changes missing coverage. Please review.

Project coverage is 27.79%. Comparing base (b0c39ae) to head (dc65341). Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
pointercrate-user-pages/src/login.rs	0.00%	31 Missing :warning:
pointercrate-user/src/auth/legacy/patch.rs	0.00%	14 Missing :warning:
pointercrate-user-api/src/endpoints/auth.rs	40.00%	6 Missing :warning:
pointercrate-user/src/auth/legacy/post.rs	81.25%	6 Missing :warning:
pointercrate-user/src/auth/legacy/mod.rs	89.58%	5 Missing :warning:
pointercrate-user/src/auth/patch.rs	0.00%	5 Missing :warning:
...te-demonlist-pages/src/account/list_integration.rs	0.00%	3 Missing :warning:
pointercrate-user-api/src/auth.rs	50.00%	3 Missing :warning:
pointercrate-user-pages/src/account/mod.rs	0.00%	3 Missing :warning:
pointercrate-user/src/auth/mod.rs	90.00%	3 Missing :warning:
... and 7 more

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #172 +/- ## ========================================== - Coverage 27.81% 27.79% -0.02% ========================================== Files 113 116 +3 Lines 8238 8243 +5 ========================================== Hits 2291 2291 - Misses 5947 5952 +5 ```

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