[3.0] Theme split (wave 4, part 2) — name the board index's sub templates instead of wrapping them

[albertlast]

Description

Part of the split of #7933, wave 4 part 2.

The board index is drawn by a template layer whose two halves each call exactly
one function:

function template_boardindex_outer_above()
{
	template_newsfader();
}

function template_boardindex_outer_below()
{
	template_info_center();
}

A layer says that something wraps around the middle of the page. Nothing here
does: the news fader, the board list and the info centre are three things one
after another. Theme::loadSubTemplates() already walks a sub_templates array,
so they can be named directly:

Utils::$context['sub_templates'] = [
	'newsfader',
	'boardindex',
	'info_center',
];

and the layer, along with both forwarding functions, goes.

Testing

The rendered page is byte-for-byte identical, as a guest and as an admin.
index.php fetched on release-3.0 and on this branch, normalised for the
session variable, the security tokens, the cache-busting query strings, the
<time> values and the cron timestamp:

	lines	result
guest	270	identical
admin	378	identical

The admin capture is a real logged-in session — it contains the logout link, the
mark-all-as-read button and the user menus — so both branches of
template_boardindex() are covered.

Notes for the reviewer

• template_main() becomes template_boardindex(). It has to have a name to
  be listed. A custom theme that overrides BoardIndex.template.php and provides
  only template_main() will stop being called, so this is worth a line in the
  upgrade notes whenever those get written. Nothing in the repository refers to
  the old name — I checked Sources/, Themes/ and SSI.php — and
  boardindex_outer has no other references either.
• The class docblock said "and main sub template", which is no longer true, so it
  now names the three.
• This is deliberately only the structural change. The theme branch also rewrites
  the markup of all three (board_container, board_info, the info centre's
  info_block / info_block_icon split, an avatar in the last-post column) and
  those need their index.css slice with them, so they follow separately.

Issues References (Fixes|Related|Closes)

Related to #7933