Feat(subscriptions/buy_minutes): add route [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

This MR adds the :new_route_ci_minutes_purchase feature flag and the /subscriptions/buy_minutes route

Screenshots (strongly suggested)

Feature flag disabled	Feature flag enabled	subscription/new

Does this MR meet the acceptance criteria?

Conformity

Changelog and documentation not added, as this introduced behind the :new_route_ci_minutes_purchase feature flag.

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

Availability and Testing

• Check this MR out
• Navigate to http://localhost:3000/-/subscriptions/buy_minutes
• You should see the 404 page
• Run bin/rails c and then Feature.enable(:new_route_ci_minutes_purchase) to enable the feature flag
• Reload the page and you should see the empty page

• 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

Related to #321639 (closed)