Provide a "rollback items" operation for feeds

Problem/Motivation

As a feeds administrator whose recent import has caused unwanted changes (updates) to entities with revisions enabled, I want to revert to the revision before the import, so that I can undo the unwanted changes.

In Feeds 7.x-2.x, it is possible to delete all nodes affected by a Feeds import using the "Delete items" tab on the import page. This useful when an importer created nodes. However, if the importer modified existing nodes, it is not desirable to delete them.

If a content type is configured so that new revisions are created by default, and Feeds updates nodes of that content type, then \FeedsNodeProcessor automatically creates a revision named "Updated by FeedsNodeProcessor" when it saves its changes.

Proposed resolution

Provide a bulk operation similar to the "Delete items" tab, which looks through each entity modified during an import, and reverts them to the revision before the "Updated by FeedsNodeProcessor" revision.

Remaining tasks

1. Collaborate with #2692945: Add ability to revert imported data (for entities that support it)
2. Write tests
3. Reviews, feedback
4. RTBC, feedback
5. Commit

User interface changes

When viewing an import form (at import/%feeds_importer), there is a "Roll back items" tab. Clicking on it presents a similar interface to the "Delete items" tab. Performing the "Roll back" operation goes through each entity modified during the import. If that entity has an "Updated by FeedsNodeProcessor" (etc.) revision, then the node is reverted to the revision before that one.

API changes

The \FeedsSource class gains the following functions:

1. \FeedsSource::startRollback(), which operates in a similar manner to startClear()
2. \FeedsSource::scheduleRollback(), which operates in a similar manner to scheduleClear()
3. \FeedsSource::rollback(), which operates in a similar manner to clear()
4. \FeedsSource::progressRollingBack(), which operates in a similar manner to progressClearing()

Data model changes

None.