Use 2-char lang code as Language primary key

[lkeegan]

• enforces uniqueness of languages
• get languages now returns a list of these lang codes
• remove langStore from frontend, instead we can now directly use the i18n-svelte library:
  • $locale for current lang_id
  • $locales for list of available lang_ids
• update frontend and admin frontend accordingly
• note: involves many changes to the database, probably easiest to just delete milestones.db
• resolves #128

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 33.33333% with 52 lines in your changes missing coverage. Please review.

Project coverage is 24.29%. Comparing base (d9d7676) to head (b566083).

Files with missing lines	Patch %	Lines
frontend/src/lib/components/Admin/Languages.svelte	0.00%	11 Missing :warning:
.../lib/components/Admin/EditUserQuestionModal.svelte	0.00%	7 Missing :warning:
...ntend/src/lib/components/Admin/Translations.svelte	0.00%	7 Missing :warning:
frontend/src/lib/components/Milestone.svelte	0.00%	7 Missing :warning:
frontend/src/lib/i18n.ts	0.00%	6 Missing :warning:
...ib/components/Admin/EditMilestoneGroupModal.svelte	0.00%	3 Missing :warning:
...src/lib/components/Admin/EditMilestoneModal.svelte	0.00%	3 Missing :warning:
...nd/src/lib/components/Admin/MilestoneGroups.svelte	0.00%	3 Missing :warning:
...tend/src/lib/components/Admin/UserQuestions.svelte	0.00%	3 Missing :warning:
...backend/src/mondey_backend/databases/milestones.py	0.00%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #129 +/- ## ========================================== + Coverage 24.24% 24.29% +0.05% ========================================== Files 95 94 -1 Lines 3853 3836 -17 Branches 122 121 -1 ========================================== - Hits 934 932 -2 + Misses 2850 2836 -14 + Partials 69 68 -1 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

[github-advanced-security[bot]]

This pull request sets up GitHub code scanning for this repository. Once the scans have completed and the checks have passed, the analysis results for this pull request branch will appear on this overview. Once you merge this pull request, the 'Security' tab will show more code scanning analysis results (for example, for the default branch). Depending on your configuration and choice of analysis tool, future pull requests will be annotated with code scanning analysis results. For more information about GitHub code scanning, check out the documentation.

[sonarcloud[bot]]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud