Disable the 'Delete revisions now' option if there not exists candidates nodes with revisions to delete

Problem/Motivation

If there not exists candidates nodes with revisions to delete you are able to launch the Delete revisions now, but this have not sense because there is nothing to delete.

Proposed resolution

Disable the option if there is not revisions to delete.

Remaining tasks

None.

User interface changes

If there not exists candidates nodes with revisions to delete the option will be disables and the description will be:

> There not exists candidates nodes with revisions to delete.

API changes

None.

Data model changes

None.