Honour the ACP "Show like counts" setting

[avandenberghe]

Closes #197.

Problem
The ACP saved "Show like counts" (rt_show_likes), but nothing read it. Like counts showed whenever Post Love was installed, whatever the setting.

Fix
core/recenttopics.php: a new show_likes() helper returns true when the Post Love service is present and rt_show_likes is on. It's used for:

• the S_POSTLOVE template flag, which every Recent Topics template checks before showing a like count;
• fetching the like counts, so switching the setting off also skips the Post Love query.

Tests

• New tests/core/topic_likes_test.php, documented in tests/tests.md, runs fill_template() with a stand-in Post Love service.
  • Setting off: no like count, no Post Love query. This failed before the fix (the count of 7 was still shown).
  • Setting on: the count of 7 is shown.
• Unit suite: 48 tests, 0 failures, run locally. The functional tests run in CI.

Checked on the local board (Post Love active, admin user, setting changed in memory only):

Code	setting on	setting off
current develop33	S_POSTLOVE = true	S_POSTLOVE = true (setting ignored)
this PR	S_POSTLOVE = true	S_POSTLOVE = false

None of the local board's recent topics have likes, so the like totals were 0 in every case. The unit test covers the counts themselves.

No changelog entry or version bump, per this repo's convention.

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