Change posts to module-based file organization

Summary:
* I would like to transfer features like tags and config as well
* moves all things related to posts to `app/posts/`

Test Plan:
* test if everyhing works as before
* test if you like the module based file organization ;)

Reviewers: rjmackay, shadowhand

Reviewed By: shadowhand

Subscribers: rjmackay, aMoniker, seth

Maniphest Tasks: T926

Differential Revision: https://phabricator.ushahidi.com/D482