Refactor to remove add_plate_modal_dialog.html and replace with use of addPlateModal.js's insertPlateModalDiv

Inserting add_plate_modal_dialog.html via template includes was the first attempt at centralizing this html/functionality. However, for JS unit testing, it turns out we really need to be able to insert this HTML via a JS call on the dom. I definitely don't like it existing in two places (as it currently does) and I never liked the template include approach anyway, so let's transition to the js dom approach. Will require (hopefully small) modification to all the pages in which add_plate_modal_dialog.html is included (which is I think all 7 of the templates that use the "add plate" functionality).