Fix Not Authorized error on SocialMedium#new
Fix specs for social media policy.

The specs were pretty badly broken. I had to make a couple fixes.

1. Authorize takes a User, not something like SocialMedium.
2. Students should be able to edit a SocialMedia if they are a permissions actor.
Merge pull request #108 from biola/fix_social_medium_policy

Fix Not Authorized error on SocialMedium#new