a11y: Set aria-label attribute for Flags in Query UI

Summary:
Input elements should have a label per https://dequeuniversity.com/rules/axe/4.10/label.
This is not the case for the <input> elements with the class `phabricator-flag-select-checkbox` on the Flag Query page. Add an `aria-label` attribute for screenreader use which simply reuses the flag color - better than nothing.

Also replace the useless repetitive `name="class[]"` for all flag elements with the flag color name.

Refs T16072

Test Plan:
* Go to http://phorge.localhost/flag/
* Click on the "Edit Query" button
* Inspect the HTML of one of the <input> items under "Colors"
* Optionally, run an Accessibility check in Chromium Lighthouse

Reviewers: O1 Blessed Committers, mainframe98

Reviewed By: O1 Blessed Committers, mainframe98

Subscribers: mainframe98, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T16072

Differential Revision: https://we.phorge.it/D26248