User proper URL for registering Feed Tags subnav item

This will allow the 'current' class to be added properly by WP's menu builder.

See #308, #166
Move Feed Tags submenu to ensure it's under PressForward menu in all cases

Adding a custom taxonomy page underneath a custom menu item that is not a CPT
can cause problems with current menu detection in WP. Filtering the value of
'parent_file' allows us to work around this.

See #308, #166
Trailing whitespace