Add a list of all templates

[bonelifer]

A new page at /wiki/templates lists every template of the wiki, linked from the overview next to Recent changes.

• One row for each Template: article, A to Z, using its live version (the newest approved one): its name, the article's description field, how many articles use it, and when it was last edited. The name links to the template's page.
• Ordinary articles and templates with no approved version are not listed. Visible to anyone who may read the wiki, 25 per page, with the usual pagination.
• The use count reuses the "What links here" logic, so it costs two queries for each row on the page.
• New strings are in the English language file only. README and the TODO plan are updated.

Tests

• Unit tests: 222 tests, 515 assertions, all pass.
• A new browser test passes 7 of 7 on phpBB 3.3.19 with SQLite, 4.0.0-a3-dev with SQLite, MySQL 8.0 and PostgreSQL 16: A to Z order, the live version deciding the description, the use counts, ordinary articles left out, the link to the template page, and the link from the overview.

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Investigated and written by Claude on behalf of William Jacoby (bonelifer).