Add financial aid view for Formstack financial aid applications
Add Django signal for Formstack webhook postbacks
Fix bug with blank api_key in FormstackAppSettingsAdmin

The admin was comparing FormstackAppSettings.api_key with None to
check for blank value, rather than the empty string (api_key is a
non-nullable char field). Fixed to compare with the empty string.
Allow FormstackAppSettings.finaid_form_id to be blank
Fix Formstack URL in template

Use form.info.url instead of fixed URL. This fixes a problem with
http/https mismatch -- Formstack freaks out if you try to access a
SSL-enabled form with http://, or a non-SSL-enabled form with
https://. The url returned by Formstack's API always has the correct
prefix, so use that instead.
Change method of creating FormstackStudentProgramApp objects

Instead of fetching the API every time we make a query for
applications, use webhooks and signals to create them as they are
submitted.

The fetch() method still exists for manually resyncing with the API.

Also fixes a bug with the get_subject mapping of strings to
ClassSubjects; the syntax "except a, b" was incorrect.