Show selected form on create post modal

Summary:
Show "Create [form name]" as the modal title when creating a post.
For example if the form name is "Missing Person" the modal title is
"Create Missing Person"

Test Plan:
Click 'Create'
Select a form
See the modal title

Reviewers: shadowhand

Reviewed By: shadowhand

Maniphest Tasks: T812

Differential Revision: https://phabricator.ushahidi.com/D367