BE: Move CI Minutes purchase flows to GitLab.com UI

Summary

This is the BE version of #1990 (closed).

Problem

During the CI minutes purchase process, depending on how the user entered the purchase flow, they may be asked which namespace the CI minutes should be applied to. This is an over complication as a result of disparate systems.

Proposal

Move the CI Minutes purchase flows to GitLab.com CTAs.

Success Criteria

• The user should remain in GitLab.com from a UI perspective
• The user should not be prompted to log into the Web Store
• The user should not be prompted to enter a namespace for the purchase

Testing & Availability

• User redirect to checkout page when clicking on buy more CI minutes on Usage Quota page
• Namespace field default to personal/group namespace
• User able to change quantity during purchase
• Invalid quantity value is not allowed
• User able to enter billing info for new payment method or use an existing one
• User able to verify order summary before proceeding to purchase
• User be prompted with successful purchase/purchase error message
• User be redirected back to Usage Quota page after purchase