Add admin test coverage and fix two letters-form crashes

[jayvarner]

Summary

Adds test coverage for the Administrate admin dashboards ahead of the planned administrate 0.20 -> 1.0 upgrade, and fixes two real bugs found while writing it - both were breaking the letters admin form in production.

• Request specs for all 9 routed admin dashboards (entities, letters, big_sams, media, faqs, about_pages, mentions, repositories, languages) - baseline auth/render coverage plus deep specs for the custom LettersController/EntitiesController logic (date-range filtering, referer-based param restoration, blank-label exclusion, e_type pre-population)
• Unit specs for the three custom Administrate::Field subclasses (HasManyThroughField, RichTextField's sanitize allowlist, ContenteditableField) and dashboard-level specs (display_resource, LetterDashboard#permitted_attributes)
• System specs (new Capybara + headless Chrome infra, mirroring Administrate's own test setup) covering the letters form's jQuery/selectize entity-tagging widget - the piece most likely to silently break across the Administrate upgrade, since no request spec executes JS

Bugs fixed

1. HasManyThroughField#associated_resource_options hardcoded order: { e_type: :asc } in its Elasticsearch query, but e_type was never mapped as sortable - this crashed the letters new/edit page for every letter. Fixed by sorting in Ruby after the search instead of asking Elasticsearch to sort.
2. The letters form has no visible submit button - saving depends entirely on the JS auto-submit-on-selectize-change behavior, which was crashing on page load (Cannot read properties of null) because of two dead lines referencing an authenticity_token field this api_only app never renders. Removed them; neither variable was ever used afterward.

Known bugs documented, not fixed

10 pre-existing bugs surfaced by the new specs are left as pending/skip with the exact root cause in each - wrong accessible_action? argument order in mentions/edit.html.erb and media/show.html.erb, a flat-vs-nested view-iteration bug in mentions/_form.html.erb, and LetterDashboard::ATTRIBUTE_TYPES missing a :published entry that breaks both show and index pages.

Test plan

• Full new spec suite green and stable across 10+ random seeds (spec/requests/admin, spec/fields, spec/dashboards, spec/system)
• Full existing application test suite green with no regressions (315 examples, 0 failures)
• Confirm the letters admin form actually saves in production after deploy (this is exactly the workflow that was broken)