R1:743dc9fdb514

Stop "Header" fields (labels for form sections) from trying to generate Conduit edits

Summary: Fixes T12236. Headers are currently trying to generate an edit transaction for `maniphest.edit` and similar, but should not, since you can't edit them.

Test Plan:
- Configured Maniphest with a custom header field.
- Before change: `maniphest.edit` API console page fataled.
- After change: all good, no weird "header" transaction.
- Header still shows up on "Edit Task" form in web UI.

Reviewers: chad…