Create a new action on the EE::Subscriptions controller for CI buying.

Part of customers-gitlab-com#2346 (closed), This issue targets on having an action inside the SubscriptionsController which targets only the work for buying the CI minutes

This action is called from the button click on Usage Quota page by the admin #300136 (closed)

Why are we doing this work

As there is already a page for new which lets user select a group to upgrade a plan. This action will allow to buy CI minutes only for either any group or for the particular group for which the user clicked on 'Buy additional minutes'

Relevant links

Main Issue - customers-gitlab-com#2346 (closed) Button Issue - #300136 (closed) UX - customers-gitlab-com#2264 (closed)

Non-functional requirements

• Documentation:
• Feature flag:
• Performance: