Add admin option to delete milestone images

[lkeegan]

• add delete button below existing milestone images in admin interface
• add delete /milestone-images endpoint
• restate /admin route for convenience during development, can be removed again later
• resolves #98

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 20.00000% with 28 lines in your changes missing coverage. Please review.

Project coverage is 27.88%. Comparing base (33d4f17) to head (c735252).

Files with missing lines	Patch %	Lines
...src/lib/components/Admin/EditMilestoneModal.svelte	0.00%	23 Missing :warning:
frontend/src/lib/components/Admin/EditImage.svelte	0.00%	2 Missing and 1 partial :warning:
...nd/src/lib/components/Admin/MilestoneGroups.svelte	0.00%	1 Missing :warning:
frontend/src/routes/admin/+page.svelte	0.00%	0 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #150 +/- ## ========================================== - Coverage 27.89% 27.88% -0.01% ========================================== Files 91 93 +2 Lines 3280 3306 +26 Branches 88 90 +2 ========================================== + Hits 915 922 +7 - Misses 2299 2316 +17 - Partials 66 68 +2 ```

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

[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