Add userid option to migrate:rollback

[seth-shaw-unlv]

GitHub Issue: Islandora/documentation#1322

What does this Pull Request do?

Adds the userid option to the Drush migrate:rollback command.

What's new?

• pulls validating the option, switching the user, and switching the user back into separate functions.
• modifies the existing functions to call the generalized ones.
• adds new functions to implement the hooks for rollback.
• Does this change require documentation to be updated? No.
• Does this change add any new dependencies? No.
• Does this change require any other modifications to be made to the repository
  (i.e. Regeneration activity, etc.)? No.
• Could this change impact execution of existing code? No.

How should this be tested?

• Try to rollback a migration using the drush command; you won't see a message about user switching and, even if you don't get an error, Fedora won't reflect the changes.
• Apply the PR.
• Do another rollback using the userid flag and the userid of an account with the fedoraAdmin role. You should see a notice with "Now acting as user" and the changes should be reflected in Fedora.
• Do the migration again, using the userid flag again, and all the stuff should show up in Fedora.

Interested parties

@Islandora/8-x-committers, esp @dannylamb. We'll tag @DonRichards 'cause he 👍 the issue.

[dannylamb]

I can confirm this works great. Tested and happy with it 👍