[WIP] Initial implementation for logging search terms

[pdelboca]

This is a simple implementation of a table to log all the search terms.

Due to initial requirements, it's necessary to log in the user_id so I'm not sure it's something we want to implement here. (I made it optional just in case).

Due to the current implementation of site_search calling other searches, we need to track if it's a site_search so we do not log it twice. I'm using flask's global object to track if the request is_site_search.

[pdelboca]

Close in favor of #2