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

Differential Revision: https://secure.phabricator.com/D20416