Add post edit form, fixes T1231

Summary:
- renamed the post form template: create -> modify
- updated locale to use the same terms for create and edit
- add post edit controller
- tweak template to work with both
- misc fixes to make it all work

Test Plan: http://ushahidi.dev/posts/edit/1

Reviewers: rjmackay, spaudanjo, aMoniker, seth

Reviewed By: seth

Subscribers: vladimir, seth

Maniphest Tasks: T1231

Differential Revision: https://phabricator.ushahidi.com/D568