Get rid of BulkForm subclasses when they just override emptySelectedMessage

Problem/Motivation

Currently the empty selected message on bulk forms is configured via means of a method on the bulk form classes:

• NodeBulkForm
• UserBulkForm
• CommentBulkForm

This is something that would be very useful for site builders to be able to configure via the UI, and both NodeBulkForm and CommentBulkForm only exist to define this message in their context.

Proposed resolution

Move the message to a config setting in the BulkForm plugin.

Remaining tasks

• Write a patch
• Write upgrade path
• Write upgrade path tests

User interface changes

User will be able to configure the empty selection message on views bulk form fields.

API changes

Deprecation of

• NodeBulkForm
• CommentBulkForm

Data model changes

New config setting for views bulk operation fields.