Show custom field values on post detail page

Summary:
Render the custom field values on a post detail page.
Use a separate views per field type to render the values.

Test Plan: - Go to a post with custom fields and see field values listed

Reviewers: eyedol, shadowhand

Reviewed By: shadowhand

Maniphest Tasks: T131

Differential Revision: https://phabricator.ushahidi.com/D33