Make birthdate optional and less precise

[charmander]

• Reduces friction when signing up
• Reduces amount of personal information stored by default
• Keeps exact birthdate private in case people don’t realize that displaying their age is equivalent to displaying their birthdate, as long as the transition is recorded

We still prevent users from displaying an age that’s inconsistent with anything they’ve done with ratings, and from changing their age once set.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 67.69231% with 21 lines in your changes are missing coverage. Please review.

Project coverage is 61.11%. Comparing base (ab64dd9) to head (1def20e).

Files	Patch %	Lines
weasyl/profile.py	60.86%	17 Missing and 1 partial :warning:
weasyl/login.py	60.00%	1 Missing and 1 partial :warning:
weasyl/journal.py	75.00%	0 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1397 +/- ## ========================================== + Coverage 60.00% 61.11% +1.11% ========================================== Files 94 94 Lines 9111 9164 +53 Branches 1641 1658 +17 ========================================== + Hits 5467 5601 +134 + Misses 3208 3111 -97 - Partials 436 452 +16 ```

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[charmander]

Thanks for the review, kfkitsune! I might try to add a few more tests before deploying this.