[ticket/11703] Make jQuery CDN switch more generic.

Config variable: load_jquery_cdn -> allow_cdn
Template variable: S_JQUERY_FALLBACK -> S_ALLOW_CDN

PHPBB3-11703
[ticket/11703] Add effectively_installed().

PHPBB3-11703
[ticket/11703] Also use empty() for T_JQUERY_LINK.

PHPBB3-11703
[ticket/11703] Extend the correct migration class.

PHPBB3-11703
Merge pull request #1569 from bantu/ticket/11703

[ticket/11703] Make jQuery CDN switch more generic.