Make the comment field mandatory when performing a batch modify

[desrosj]

Batch modifications can be pretty confusing or alarming because it's not easy to understand why something was changed. What changed is detailed appropriately in the email and comment left on the updated tickets, but unless a comment is included in the field, the why is not.

When performing a batch modify, the comment field should be required so that contributors are forced to add context and justification to clarify why the modifications are being made.

[dd32]

Unlike #8044, enforcing this through our JS to add a required attribute is simple enough and would solve both tickets.

[dd32]

In 14505:

Trac: Mark the Batch Modification comment field as required.

Props desrosj, dd32.
See #8044.
Fixes #8045.