[CI Minutes] Show the banner after successful purchase

What does this MR do?

This MR is part of a sequence that addresses #321645 (closed). We're:

• providing the redirect URL to the frontend to be sent in the purchase request
• updating the data and logic to account for this case
• updating the tests

Add backend preparatory logic and banner	!66901 (merged)
Add frontend logic to show banner after successful purchase	👈 you are here

Note that:

• the feature is behind a feature flag
• the new path to purchase subscriptions is not yet exposed

Local test (it's a bit tricky... ):

First:

• make sure your local CDot instance is running
• make sure your Zuora API Sandbox Page points to your GDK URL (like in point 5.3 in here)

Then:

1. Create a new Group with a Free plan

2. within the Rails Console, enable the new_route_ci_minutes_purchase feature flag (see here for enabling FF in local dev) running Feature.enable(:new_route_ci_minutes_purchase)

3. navigate to /-/subscriptions/buy_minutes/?selected_group=<your-group-id>

4. insert data to complete the purchase

   • for the credit card, you can use one of the cards listed here

You should be redirected to the Usage Quotas page.

Screenshots or Screencasts (strongly suggested)

Screen_Recording_2021-07-26_at_09.42.34

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• 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 #321645 (closed)