UI for share rollback

Add view and template for share rollback
Make rollback api call
Move error message div to modal
Merge branch 'issue#308_rollback' of git://github.com/sujeetsr/rockstor-core into sujeetsr-issue#308_rollback
Update snapshot model to include real_name along with (display)name.
Remove share rollback from snapshot view.
Implement rollback api as part of the share command view
update shares cli to add support for rollback.
add migration file for updated snapshot model
Don't allow share rollback while it's nfs exported or shared via samba.
Correct api and fix cancel method for share rollback
Modify confirm message for rollback
Merge branch 'issue#308_rollback' of github.com:sujeetsr/rockstor-core into issue#308_rollback
Disable rollback button if there are no snapshots
Merge pull request #312 from sujeetsr/issue#308_rollback

Issue#308 rollback