* 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`
