ensure that the form object passed to enableInput has access to find and...

This PR no longer assumes that the `form` argument that's passed to `enableInput` is generated using a jQuery selector. When it isn't  (e.g. when a click or change handler calls `enableInput`), we can explicitly set `form` using a selector so it has access to the `find` and `prop` methods.

#bug
/cc @pdeuter, @zendesk/quokka

### References
[ZD#726543](https://support.zendesk.com/agent/#/tickets/726543)

### Risks
 - None