Refactor horizon/common/_modal_form.html template

Template _modal_form.html has an identical header with
_modal.html - thus it makes sense to inherit _modal_form.html
from _modal.html to reduce duplication.

Partially Implements: blueprint form-template-to-view
Change-Id: Idf0c9b14712bc1c433cb6a5aef602d1820af915c
Merge "Refactor horizon/common/_modal_form.html template"