[Delivers #47583811] enhance the linker browse modal to include sort and facets

So I've introduced filter and sorting to the linker browse modal by pulling in the generic search results template and overriding all the link clicks to become ajaxified. There's now a three 'formats' available to the search action - "json" for results in json, "html" for the full page with layout etc, and "js" to support the ajax loading of the results html.

There were plenty of linker tweaks.. 
- it needs both a url for the typeahead (json) and a different for the browse search (returns HTML);
- introduced I18n for the linker strings
- disable linkers opened in the create modal (they currently don't play nice)
- the linker now assumes all records will offer a 'title' ... which was almost true for all except for agents.. until now...
- introduced a readonly 'title' property on agents which offers their first name's sort_name
- exclude the current agent from the related agent linker browse list