Rename 'facet' to 'filter' in frontend code

Currently all of the inputs on search form are derived from a schema and called 'facets'. They are styled using the class 'facet'. This is about to change as we are adding keyword search which will search across facets but is not in and of itself a facet.

This refactor removes the term 'facet' from CSS and JavaScript and replaces it with 'filter' which is a more general term that encapsulates facets and keyword searches. Using "filter" allows sharing of styling without having to pretend keyword-search is a facet.