Move Storage purchase flows to GitLab.com (Group namespaces only)
## Proposal
Move the GitLab.com storage purchase flow to GitLab.com CTA.
## Designs
See [UX issue](https://gitlab.com/gitlab-org/customers-gitlab-com/-/issues/2265)
## 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
## Release Blocker
This feature should be released behind a feature flag until https://gitlab.com/gitlab-org/gitlab/-/issues/270424 is deployed, however, the work can be done in parallel.
## Updated Timeline
| Milestone | Changes | Status |
| ------ | ------ | ------ |
| %"14.6" | Dev work | :white_check_mark: |
| %"14.7" | **Dev work**<br/>1. Improve UX for Storage purchase via GitLab https://gitlab.com/gitlab-org/gitlab/-/issues/347331<br/>2. Storage Purchase in GitLab does not account for pro-rated case https://gitlab.com/gitlab-org/gitlab/-/issues/347393 | :white_check_mark: |
| %"14.8" (February 1, 2022) | **Launch** - Storage purchase in gitlab.com https://gitlab.com/gitlab-org/gitlab/-/issues/327896 | :white_check_mark: |
| %"14.8" | **Cleanup work:** <br/>1. [Storage] [Feature flag] Remove the 'new_route_storage_purchase' feature flag https://gitlab.com/gitlab-org/gitlab/-/issues/337720 | :white_check_mark: |
epic