Twenty Thirteen: Use core function for featured galleries

[obenland]

Themes, especially default themes should not deal with regex in functions.php.
Let's move the functionality into core and just use a filter.

In 24029:

Twenty Thirteen: Use the_post_format_gallery(). props obenland, fixes #24126.

In 24033:

Twenty Thirteen: Don't use the_post_format_gallery() as it does not exist (yet). see #24126.

Patch works for Twenty Thirteen as expected!

Patch tested out well for me, also.

@nacin Can you review and commit, please?

Still needs another look and commit from Nacin.

In 24093:

the_post_format_gallery() (and Twenty Thirteen using it).

fixes #24126. props obenland, wonderboymusic.