Dialog box on trial generation error

What does this MR do?

This change is part of the fix that prevents multiple trials.

1. The GitLab instance passes the email ID of the current user as part of query params. The subscription portal shows this email ID as read only. This is to reduce the attempts of users who may use different email IDs every time to request a trial.

2. If the license app returns an error response, we redirect back to the GitLab instance with license_key = nil. And show a modal with the message:

3. The license app in the background sends an email prompting the user to reach out to sales.

Related MRs:

Customer app: https://gitlab.com/gitlab-org/customers-gitlab-com/merge_requests/382

License app: https://gitlab.com/gitlab-org/license-gitlab-com/merge_requests/99

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance 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

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