Prevent users from removing task titles with "Bulk Edit"
Summary:
See downstream <https://phabricator.wikimedia.org/T209449>.
The "Bulk Edit" flow works with `setContinueOnMissingFields(true)`, so `newRequiredError()` errors are ignored. This allows you to apply a transaction which changes the title to `""` (the empty string) without actually hitting any errors which the workflow respects.
(Normally, `setContinueOnMissingFields(...)` workflows only edit properties that can't be missing, like the status of an object, so this is an unusual flow.)
Instead, validate more narrowly:
- Transactions which would remove the title get an "invalid" error, which is respected even under "setContinueOnMissingFields()".
- Then, we try to raise a "missing/required" error if everything otherwise looks okay.
Test Plan:
- Edited a task title normally.
- Edited a task to remove the title (got an error).
- Created a task with no title (disallowed: got an error).
- Bulk edited a task to remove its title.
- Before change: allowed.
- After change: disallowed.
Reviewers: amckinley
Reviewed By: amckinley
Differential Revision: https://secure.phabricator.com/D20339