Add 'Add Seats' Button to Billing section

What does this MR do?

Relates to gitlab-org/customers-gitlab-com#2305, this is a series of MR planning to add a button to give customers the possibility to add more seats. The reason of separating it is the complications arising from the multi=project handling plus the fact that it should be faster getting merged 🤞

The Plan

Description	MR
backend Add the Feature Flag and the new URL	!49242 (merged)
backend Redirect for the new URL (on CustomersDot)	gitlab-org/customers-gitlab-com!2307
frontend Add the Button on the Frontend	👈 You are here!
frontend Refactor injected vars and tests	tbd
Roll-out the feature	tbd

Changes

This MR adds the button and the tests to support the implementation. Note that this is behind a feature flag so the button does not show by default. For the same reason, the changelog will be added with the Roll-out Issue.

Test steps

Follow these steps with feature flag name :saas_add_seats_button.

Screenshots (strongly suggested)

Feature Flag Disabled	Feature Flag Enabled

Does this MR meet the acceptance criteria?

Conformity

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

Availability 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
• [-] 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