Remove the "willApplyTransactions()" hook from ApplicationTransactionEditor

Summary:
Depends on D19908. Ref T13222. In D19897, I reordered some transaction code and affected the call order of willApplyTransactions().

It turns out that we use this call for only one thing, and that thing is pretty silly: naming the raw paste data file when editing paste content.

This is only user-visible in the URL when you click "View Raw Paste" and seems exceptionally low-value, so remove the hook and pick a consistent name for the paste datafiles. (We could retain the name behavior in other ways, but it hardly seems worthwhile.)

Test Plan:

• Created and edited a paste.
• Grepped for willApplyTransactions().

Note that EditEngine (vs ApplicationTransacitonEditor) still has a willApplyTransactions(), which has one callsite in Phabricator (in Calendar) and a couple in Instances. That's untouched and still works.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13222

Differential Revision: https://secure.phabricator.com/D19909