Validate user sessions on bulk upload when f2f type is single

[rhell4]

Currently when using the bulk upload feature a user can be added to multiple sessions when the f2f signup type is set to single.

This adds validation checking on both existing signed up sessions and multiple sessions for a single user in a bulk upload.

Testing:

• F2F activity with signup type set to single
• 3 sessions added to the F2F
• Single student enrolled in the course
• Signup the student to session 1
• Upload a CSV to bulk signup student to session 2 and 3
• You should see the following errors
  • Student is already signed-up for this Face-to-Face activity.
  • User student has more than one session
• Edit F2F and change the signup type to multiple
• Try uploading the same CSV file again
• You should see "Uploaded file has been validated and ready to be processed."
• Click "Confirm and process"
• You should see "Successfully processed records."
• The student should be signed up for all 3 sessions now