Make sure custom language files (if they exist) are taking precedence over GlotPress language packs

There can be cases where a community needs cutom language files. In order to make sure these custom language files are loaded, BuddyPress will first check 2 locations before loading the GlotPress language packs :
- {{{WP_LANG_DIR . '/buddypress'}}}
- {{{WP_LANG_DIR}}}

Using the {{{WP_LANG_DIR . '/buddypress'}}} location for the custom language files avoids the mo file name to be listed into the site's language setting of the WordPress general options.

We are also introducing a new filter {{{'buddypress_locale_locations'}}} so that it's possible to add a custom location to the ones listed above.

Props r-a-y, boonebgorges,danbp,DJPaul

Fixes #5887

git-svn-id: https://buddypress.svn.wordpress.org/trunk@9247 cdf35c40-ae34-48e0-9cc9-0c9da1808c22