Show HTML entities in article text as their characters

[bonelifer]

Fixes #66. As on MediaWiki, an entity written in a wiki article now shows as the character it stands for. Before, that only happened inside a template's output.

• In the text: s9e's HTMLEntities plugin is loaded, and its tag is switched on only while wiki markup is (wikilink_listener::toggle_wiki_markup()), like the wiki's other tags.
• In captured text: headings, link texts and targets, category names and file alt texts are captured in one piece, and the plugin never sees them. An attribute filter, markup::decode_entities(), decodes them at the same moments. In a link target or category name, a non-breaking space counts as a space, as in MediaWiki page names.
• Safe: the character is output as text, so <b> shows as "" and never becomes markup. An unknown entity, or one for a control character, stays as written.
• Not in forum posts: both only apply while wiki markup is on, so posts, [[links]] in posts included, show entities as written, as before.
• Not twice in templates: template text has already been decoded once on its way to the parser, so templates::render() and page_view() switch decoding off (markup::set_entities()). < in a template still shows "<".
• Existing articles show their entities as written until they're saved again or reparsed. The README says so, next to the other markup.

Claude tested this through the edit form's preview on a seed-forum wiki test board (phpBB 3.3):

• A B, A B and A—B show their characters, in text, a heading, a link's text and a link's target;
• <b> and < show as "" and "<", and &bogus; stays as written;
• a forum post keeps its entities as written;
• < in a template is not decoded twice.

Two new unit tests. The full suite passes on phpBB 3.3 and master with run-unit-tests.sh. The 30 examples in phpbbmodders/wiki-templates still pass on this branch.

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

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