Generation of modal titles specific to document type. #62665250

Per [story 62665250](https://www.pivotaltracker.com/story/show/62665250).  Redo of [PR 859](https://github.com/reciprocity/ggrc-core/pull/859).  In each URL field of a PBC Response, the `data-lookup` attribute is replaced with a class derived from `GGRC.Models.Document` that replaces the `.title_singular` attribute with the more specific name.  This is then passed through to the document autocomplete template.