more moderation features

[kmahar]

• added some extra post metadata fields to schema - auto migration should be fine.
• depending on whether approve or reject is selected, show mod different additional options
  • if accept, mod can add sender to whitelist
  • if reject, mod can add sender to blacklist, write an explanation, and/or add (for now) generic tags about content
• backend support for the tag and explanation features
• cleaned up duplicate logging statements (maybe from an auto-merge) in views.py

[amyxzhang]

@kmahar can you update given the conflicts? Thanks!

[kmahar]

should be good now!