search: Renamed weighted_ilikes list to expressions
search: Moved search code to model.base

Every table that has a __searchable_columns__ attribute can now be
searched with the new algorithm.
search: Extracted combined_search_query() function
search: Added optional include_missed parameter
search: Moved functions to the top
search: Import cleanup
search: Moved combined_search_query() to new model.search module
search: Extracted escape_tokens() function
search: Converted models to constant MODELS list
search: Improved output