[ticket/11345] Change font for buttons

Change font to Open Sans

PHPBB3-11345
[ticket/11345] Add Droid Sans as fallback font

PHPBB3-11345
[ticket/11345] Move css import to template

PHPBB3-11345
[ticket/11345] Wrap remote webfonts in S_ALLOW_CDN.

PHPBB3-11345
[ticket/11345] Don't include scheme, so HTTPS is used where necessary.

PHPBB3-11345
[ticket/11901] Pass empty string as relative_url for non-local magic URL.

Only the regular expression for MAGIC_URL_LOCAL has three capturing groups. As
a result $matches[3] is only set in this case. Prior to PHPBB3-11606 an empty
string was passed to make_clickable_callback() as a replacement in case of less
than three capturing groups. This patch reintroduces the empty string.

Regression from 49c12ef4be229bf2223139298766ef441b075fbc.

PHPBB3-11901
Merge pull request #1763 from bantu/ticket/11901

[ticket/11901] Pass empty string as relative_url for non-local magic URL...
[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
[ticket/11903] Speed up paging test

PHPBB3-11903
Merge pull request #1765 from EXreaction/ticket/11903

[ticket/11903] Speed up paging test
Merge pull request #1569 from bantu/ticket/11703

[ticket/11703] Make jQuery CDN switch more generic.
Merge pull request #1571 from bantu/ticket/11345

[ticket/11345] Better font for buttons
Merge remote-tracking branch 'phpbb-core/develop'

* phpbb-core/develop:
  [ticket/11903] Speed up paging test
  [ticket/11703] Extend the correct migration class.
  [ticket/11703] Also use empty() for T_JQUERY_LINK.
  [ticket/11703] Add effectively_installed().
  [ticket/11703] Make jQuery CDN switch more generic.
  [ticket/11901] Pass empty string as relative_url for non-local magic URL.
  [ticket/11345] Don't include scheme, so HTTPS is used where necessary.
  [ticket/11345] Wrap remote webfonts in S_ALLOW_CDN.
  [ticket/11345] Move css import to template
  [ticket/11345] Add Droid Sans as fallback font
  [ticket/11345] Change font for buttons