Fix revoking of authorized OAuth applications

Users were not able to revoke access to authorized OAuth applications. Clicking the "Revoke" button would result in a 404 page, and the application would still be authorized.

Added a spec and also found that the `gon` variables were not being set for this view.

Closes #14370

See merge request !3690
Signed-off-by: Rémy Coutable <remy@rymai.me>