Fix attaching users to new posts

Summary:
- Save current user id as owner when creating a new post
- Hide email and name fields on post create form if user is logged in.
- Additional tests for assigning posts to current user
- Refactor API code to unpack related IDs
- Catching state errors from JSO when trying to use implicit flow
- Refactor post user saving

Test Plan:
- Create a new user
- Log in
- Create a post
- Check that you can view the post
- Check the post is owned by the current user (user_id in db matche current users id)

Reviewers: shadowhand

Reviewed By: shadowhand

Maniphest Tasks: T147

Differential Revision: https://phabricator.ushahidi.com/D15