forms: Fix poor form rendering on new ticket by staff

Also fix a crash when re-rendering the new ticket by staff page after a
validation error.
forms: Fix warnings when creating a new custom form
oops: Partially revert e5d61922d

The real bug was setting the source of the ticket form — not the fallback to
the form entry source.  Falling back breaks edits.
Merge pull request #1380 from greezybacon/issue/1376

Fix crash submitting new ticket with validation errors

Reviewed-By: Peter Rotich <peter@osticket.com>