* Implement custom study on backend
* Switch frontend to backend custom study
* Skip typecheck for new pb classes
* Build tag search string on backend
Also fixes escaping of special characters in tag names.
* `cram.cards` -> `cram.card_limit`
* Assign more meaningful names in `TagLimit`
* Broaden rustfmt glob
* Use `invalid_input()` helper
* Assign `FilteredDeckForUpdate` to temp var
* Implement `SearchBuilder`
* Rewrite `custom_study()` with `SearchBuilder`
* Replace match macros with `SearchBuilder`
* Remove `into_nodes_list` & `concatenate_searches`
So, this is fun. Apparently "DeckId" is considered preferable to the
"DeckID" were were using until now, and the latest clippy will start
warning about it. We could of course disable the warning, but probably
better to bite the bullet and switch to the naming that's generally
considered best.
