Tags overhaul [DB reupgrade]

- Simplified schema
- Tags are now added without reloading entire tag selector
  - On my system, adding 400 tags to an item (separately, with the tag
    selector updating each time) went from 59 seconds to 42. (Given that
    it takes only 13 seconds with the tag selector closed, though,
    there's clearly more work to be done.)
- Tag selector now uses HTML flexbox (in identical fashion, for now, but
  with the possibility of fancier changes later, and with streamlined
  logic thanks to the flexbox 'order' property)
- Various async fixes
- Tests

test/tests/supportTest.js

@@ -77,7 +77,7 @@ describe("Support Functions for Unit Testing", function() {
 
 			let tags = data.itemWithTags.tags;
 			for (let i=0; i<tags.length; i++) {
-				let tagID = Zotero.Tags.getID(zItem.libraryID, tags[i].tag);
+				let tagID = yield Zotero.Tags.getID(tags[i].tag);
 				assert.ok(tagID, '"' + tags[i].tag + '" tag was inserted into the database');
 				assert.ok(zItem.hasTag(tags[i].tag), '"' + tags[i].tag + '" tag was assigned to item');
 			}