Fold #215, #216 and #217 into the 3.0.12 changelog

[avandenberghe]

3.0.12 now also ships the three fixes from the customisations team's 3.0.11 review notes (#215, #216, #217). There's no version bump: ext.php, composer.json and the README stay at 3.0.12.

contrib/CHANGELOG.md, 3.0.12 section

• The Fix last-post screen reader label #192 line claimed the last-post screen-reader label was fixed, which was true for one template only. It now says Fix last-post screen reader label #192 fixed the index top/bottom template (thanks karelpak1) and Last-post link has an empty screen-reader label in 7 templates (VIEW_LATEST_POST is never assigned) #215 fixed the other seven.
• New fix lines for UCP shows no Recent Topics section to a member who only has u_rt_number #216 (UCP section for u_rt_number-only members) and ACP "Exclude topics" silently ignores invalid input and can't be cleared #217 ("Excluded topic IDs" can be cleared; invalid input shows an error).
• The tests line adds the excluded-topics input and template tests.

After merge and green CI

1. Move the v3.0.12 tag to the merge commit.
2. Rebuild Staging/RecentTopics_3.0.12.zip and its folder from the tag.
3. Replace the GitHub release asset and notes.
4. Update the forum reply.
5. Re-sync the local board.

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