Remove default post types for showing share links

Default post_types for showing share links were being set on line 204 but then were being overwritten. This updates the default to an empty array (which is the default when the global options have not been set) but now, the show parameter can be updated by the 'sharing_default_global' filter without being reset to an empty array.
Merge remote-tracking branch 'upstream/master'
Manual merge to update with upstream master
Add filters to feedback fields and field headers.
Merge pull request #179 from tannerm/feedback-download-filters

Feedback download filters