make all calls to back_to_main explicit and remove unused params

I did this by copy pasting

```python
back_to_main(req, domain, app_id=app_id, module_id=module_id, form_id=form_id, unique_form_id=unique_form_id, edit=edit, error=error, page=page)
```
to replace every `back_to_main(**locals())` one by one, letting PyCharm highlight the variables that weren't in scope, and deleting them.

There was an issue this where PyCharm does not recognize that `del var` deletes `var` from the scope, so after doing the replacements above, I searched for "del " and removed both the `del` statement and the corresponding `back_to_main` param in all cases where I was using `del` to remove a variable from `locals()`.

This process plus some manual testing gives me high confidence that I didn't screw anything up / change the behavior at all.