Make post form attachments accessible

[K4rakara]

What

Adds a role="button", tabindex, and v-on:keydown to MkPostFormAttaches.

Why

Without this, users who are navigating the UI with a keyboard cannot add alt text to media, mark media as sensitive, or even remove media from the post they are composing.

Checklist

• [x] Read the contribution guide
• [x] Test working in a local environment*
• [x] (If needed) Add story of storybook
• [x] (If needed) Update CHANGELOG.md
• [x] (If possible) Add tests

*I've tested the parts I've changed in the browser, and they work, but for some reason I can't get pnpm run test to pass due to something with _misskey_content. I assume this is a misconfiguration of the test environment on my part (I just copied the default.yml to test.yml), but I'm not sure how.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 19.53%. Comparing base (e9519b0) to head (76d95f2). Report is 2 commits behind head on develop.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #14666 +/- ## ============================================ - Coverage 41.40% 19.53% -21.88% ============================================ Files 1549 716 -833 Lines 199468 101501 -97967 Branches 3623 987 -2636 ============================================ - Hits 82595 19826 -62769 + Misses 116276 81129 -35147 + Partials 597 546 -51 ```

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[K4rakara]

Forgot to add- this is based on this Sharkey MR: https://activitypub.software/TransFem-org/Sharkey/-/merge_requests/638

[syuilo]

🙏🏿