Add feed attribute

[itsthejoker]

Relevant issue: #174

Description:

Add a feed attribute that contains the subreddit name. This will automatically be populated on all new submissions, but will need to be populated manually by SSH'ing into the server and and running:

from blossom.api.models import Submission
all = Submission.objects.filter(feed__isnull=True)
for s in all:
  s.save()

Checklist:

• Code Quality
• Tests (if applicable)
• Inline Documentation