Resolve "NoMethodError: undefined method `oauth_applications' for nil:NilClass"

What does this MR do?

Fixes #204729 (closed)

Sentry: https://sentry.gitlab.net/gitlab/gitlabcom/issues/1215315/

Cause:

We override the set_application method in Doorkeeper gem in our Oauth::ApplicationsController.

But, our override requires the presence of current_user to be able to find current_user.oauth_applications.find(params[:id])

In the absence of a session, current_user will be nil and this will throw an error (as seen on Sentry)

Hence, we need to make sure that a session is present before the before_action :set_application filter from Doorkeeper kicks in, and hence the need to use prepend_before_action :authenticate_user!

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team