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

resource/schema/triggers.sql

@@ -210,24 +210,6 @@ CREATE TRIGGER fku_itemNotes
   END;
 
 
--- itemTags libraryID
-DROP TRIGGER IF EXISTS fki_itemTags_libraryID;
-CREATE TRIGGER fki_itemTags_libraryID
-  BEFORE INSERT ON itemTags
-  FOR EACH ROW BEGIN
-    SELECT RAISE(ABORT, 'insert on table "itemTags" violates foreign key constraint "fki_itemTags_libraryID"')
-    WHERE (SELECT libraryID FROM tags WHERE tagID = NEW.tagID) != (SELECT libraryID FROM items WHERE itemID = NEW.itemID);---
-  END;
-
-DROP TRIGGER IF EXISTS fku_itemTags_libraryID;
-CREATE TRIGGER fku_itemTags_libraryID
-  BEFORE UPDATE ON itemTags
-  FOR EACH ROW BEGIN
-    SELECT RAISE(ABORT, 'update on table "itemTags" violates foreign key constraint "fku_itemTags_libraryID"')
-    WHERE (SELECT libraryID FROM tags WHERE tagID = NEW.tagID) != (SELECT libraryID FROM items WHERE itemID = NEW.itemID);---
-  END;
-
-
 -- Make sure tags aren't empty
 DROP TRIGGER IF EXISTS fki_tags;
 CREATE TRIGGER fki_tags