Add configuration option for when to execute the handler

Problem/Motivation

Currently, the handler sends API calls on every postSave event, not only when creating or updating a submission, but also for drafts being created or updated, or when switching pages.

Steps to reproduce

Create a webform with drafting enabled and save a submission draft. Notice a CiviMRF call being made.

Proposed resolution

Add configuration options for deciding when to send API calls, similar to what the E-Mail handler does in its advanced settings. Those options should include all states (incl. delete events).

Remaining tasks

Maybe add tests or thoroughly test manually.

User interface changes

There is a new configuration option in the Advanced tab for selecting the events that are supposed to initiate API calls.

API changes

None.

Data model changes

New (implicit) configuration option states

Comments

This is Major because it makes drafting or multi-page webforms unusable without this option.