Revamp the OAuth2 application admin form

Review Request #8987 — Created June 2, 2017 and submitted — Latest diff uploaded

Information

Review Board
release-3.0.x

Reviewers

The OAuth2 application form now makes use of some new widgets to make
the user experience better. The logic for the form validation has also
been revamped to take into account when validation of the redirect URIs
field fails (since they will no longer be present in the form's cleaned
data).

Manually tested the form.
Ran unit tests.