Allow theme to override site template pack choice

[DJPaul]

As part of the work for Nouveau, we need a way to let a theme override the site template pack setting.
e.g. new themes may force the "nouveau" template pack, or vice-versa, etc.

[DJPaul]

1.patch proposes support for add_theme_support("buddypress-use-XXX"), where XXX is the ID of any registered template pack.

e.g.
`
add_theme_support("buddypress-use-nouveau");
add_theme_support("buddypress-use-legacy");
`

If one is set, and a template pack with that ID exists, then BuddyPress will load that instead of the site preference.

[DJPaul]

cc @hnla for comments

[DJPaul]

Forget 1.patch. Load order issues. Updating it, same principle though.

[DJPaul]

There we go! 2.patch.

[hnla]

Will Patch and test when have a minute.

We seem to have an issue with Trac not seeming to post notifications, at least not to slack where I normally initially pick up on ticket comments, think email pings might also be failing?

[hnla]

@djpaul Initial test looks good, forced my Nouveau test bed back to using bp-legacy templates.

Wondering what tests/scenarios we need to think about.

[djpaul]

In 11936:

Core: allow theme to override site's template pack choice.

As part of the work for Nouveau, we need a way to let a theme override the site template pack setting.
e.g. new themes could force the "nouveau" template pack.

The syntax is buddypress-use-foobar, where "foobar" is the slug name of the template pack.
e.g. buddypress-use-legacy, buddypress-use-nouveau.

Fixes #7730