Hide placeholder images unless media uploads feature is enabled

Summary:
Refs T79
Refs T161

Test Plan:
Go to post listing - no images
Go to post detail - no images
Enable media upload feature
Go to post listing - see images
Go to post detail - see images

Note this creates some UI issues when images are missing. These needs resolving
since not all posts have images anyway.

Reviewers: seth, dkobia, shadowhand

Reviewed By: shadowhand

Subscribers: srutto

Maniphest Tasks: T79, T161

Differential Revision: https://phabricator.ushahidi.com/D206