Change purchase url to CustomersDot when you are on trial

Issue link: https://gitlab.com/gitlab-jh/gitlab/-/issues/452

Depended MR: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/77435

What does this MR do

When the user is in the trial period, there will be a button to buy.

We need to change the href of this button to the CustomersDot url, instead of http://GitLabHost/-/subscriptions/new .

A valid CustomersDot url looks like:

http://CustomersDotHost/subscriptions/new?plan_id=2c92c0f97700a4f70177060963d923c8&gl_namespace_id=117

The CustomersDot url is fetched from CustomersDot API, request path: /gitlab_plans.

If the value is not available, we will let the user go to CustomersDot home page.