Add X-Delete-At/After support to FormPost

This change adds the ability to specify a X-Delete-At or
X-Delete-After attribute when using the FormPost middleware.

Here is an example of what you need to add to the form:

  <input type="hidden" name="x_delete_at" value="<unix-timestamp>"/>
  <input type="hidden" name="x_delete_after" value="<seconds>"/>

To be inline with the other form imput names, x-delete-at/after has
changed to x_delete_at/after.

DocImpact
Change-Id: Ib1cc0bcf1dd7d2b689f2f26d100f9bab36880c81
Closes-Bug: #1065522
Merge "Add X-Delete-At/After support to FormPost"