R1:9532bfbb32c8

Improve trigger editor behavior when switching to/from tokenizers

Summary:
Ref T13269. See D20329. When we switch trigger rule control types, reset the rule value.

Also, pick slightly nicer defaults for status/priority.

Test Plan:
- Created a "Change Status To: X" rule.
- Saved it.
- Edited it.
- Selected "Assign to" for the existing action's dropdown.
- Before: tokenizer filled with nonsense.
- After: tokenizer cleared.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13269…