Search Subsites

WIP #5481

This still needs:
* Change of the 'other websites' field to autocomplete
* Change popup title to "Search Settings"
I don't think using this link is a good idea. `os_search_solr` is not app (no `hook_os_app_info` for it, and it is not configurable per vsite).
On the other hand, we need to create this per vsite solr search settings page, and make it accessible from somewhere.