Refactor bits of advanced search

This commit never made it to develop/release branch because it was merged into a feature branch that was already merged at the time. https://github.com/reciprocity/ggrc-core/pull/1633
References to Workflow or workflow object types should always stay in ggrc_workflows/ (unless there's a really good reason for them not to be).

@danring is of the opinion, furthermore, that since this is likely a band-aid on advanced search, this mapping group and the associated model classes (MultitypeSearch and MultitypeSearchJoin) should be bundled into advanced_search_controller.js with clear comments that say that this should be improved upon ASAP (i.e. lots of FIXMEs).  I am on board with this.