feat: implement user tags • part 1

[AkesiSeli]

Technical details

This PR contains the persistence and domain layers needed to implement user tagging as tracked by #195.

Additional notes

Future PRs will contain the UI and the business logic needed to manage tags and membership.

[codecov]

Codecov Report

Attention: Patch coverage is 95.08197% with 3 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...persistence/repository/DefaultUserTagRepository.kt	94.64%	0 Missing and 3 partials ⚠️

Flag	Coverage Δ
unittests	56.43% <95.08%> (+0.36%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
...rlemmy/core/persistence/data/UserTagMemberModel.kt	100.00% <100.00%> (ø)
...ersistence/repository/DefaultSettingsRepository.kt	71.63% <ø> (ø)
...persistence/repository/DefaultUserTagRepository.kt	94.64% <94.64%> (ø)