VID-890

**TL;DR**
* Namespaces were hard-coded, even though the default content namespaces of a wiki are configurable
* This was breaking video search
* The video filter was also missing from video search, so this is also being fixed here

## Summary ##
This addresses the limitations on header search form namespace hidden field configuration per #1462 (PLA-670) by utilizing logic already available in the WikiaSearch extension. 

The required logic for populating the appropriate namespaces for a given wiki has been ported to a trait, Wikia\Search\Traits\NamespaceConfigurable. This allows the controller responsible for handling search pages as well as the controller responsible for populating the global search form to use the same logic for namespace filtering on either search form. 

Additionally, default video search requires a default behavior of filtering for videos. This seems to have been lost somewhere along the line. The updates included here allow us to provide this filter when on the Video wiki. So this also fixes a somewhat related existing regression.

This fix doesn't just repair the problems with the video wiki, but any wiki that has different default content namespaces.