Add milestones to admin interface

[lkeegan]

• admin interface can now add/edit/delete milestones
• fix bug in backend regarding milestone image filenames
• add development readme

[sonarcloud[bot]]

Quality Gate passed

Issues
3 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

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0.92593% with 214 lines in your changes missing coverage. Please review.

Project coverage is 18.23%. Comparing base (760f859) to head (ea38aa9).

Files with missing lines	Patch %	Lines
frontend/src/lib/admin.ts	0.00%	95 Missing :warning:
...src/lib/components/Admin/EditMilestoneModal.svelte	0.00%	77 Missing and 1 partial :warning:
...nd/src/lib/components/Admin/MilestoneGroups.svelte	0.00%	40 Missing :warning:
frontend/src/routes/admin/+page.svelte	0.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #97 +/- ## ========================================== - Coverage 19.19% 18.23% -0.96% ========================================== Files 83 84 +1 Lines 3725 3921 +196 Branches 112 113 +1 ========================================== Hits 715 715 - Misses 2951 3146 +195 - Partials 59 60 +1 ```

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