Delete revision by life time

Problem/Motivation

As described in https://www.drupal.org/node/1786720:

> Will be good to see in new versions feature for deleting old revisions by time, not by count. For example, remove all revisions older that 1 month from current time.

Proposed resolution

Use a similar approach used in the issue (https://www.drupal.org/node/1786720) and in the [Revision Deletion](https://www.drupal.org/project/revision_deletion) module

Remaining tasks

None.

User interface changes

A new option will appears in the content type edit page.

API changes

None.

Data model changes

The config file will be updated with a new variable.