Enable storage purchase link

What does this MR do?

Adds a link to EE::SUBSCRIPTIONS_MORE_STORAGE_URL for the big ol' green "Purchase more storage" button.

Since EE::SUBSCRIPTIONS_MORE_STORAGE_URL is optionally provided based on the buy_storage_link feature flag, we also conditionally render the UsageStatisticsCard containing the big ol' green button.

Note that in almost all cases the buy_storage_link feature flag will be on here.

• No changelog as this is behind the disabled additional_repo_storage_by_namespace feature flag

Screenshots (strongly suggested)

buy_storage_link disabled	buy_storage_link enabled

Testing this locally

Requires EE edition

Paste the following in your Rails console

# Match the 10GB limit on GitLab.com, enable application setting
ApplicationSettings::UpdateService.new(Gitlab::CurrentSettings.current_application_settings, nil, { repository_size_limit: 10240, enforce_namespace_storage_limit: true }).execute

Feature.enable(:additional_repo_storage_by_namespace)
Feature.enable(:buy_storage_link)

ACCESS_LEVEL_OWNER = 50
PACKAGE_SIZE_IN_MIB = 10240

def create_group_with_usage_and_additional_storage(owner, group_name, nr_of_packs_purchased, usage)
  group = Group.create(name: group_name, path: group_name, type: 'Group')
  GroupMember.add_user(group, owner, ACCESS_LEVEL_OWNER)
  project = ::Projects::CreateService.new(owner, { namespace_id: group.id, name: 'project-' + group_name, path: 'project-' + group_name }).execute

  group.additional_purchased_storage_size = nr_of_packs_purchased * PACKAGE_SIZE_IN_MIB
  group.save!

  project.statistics&.destroy!

  project_statistics = ProjectStatistics.new(
    project_id: project.id,
    namespace_id: group.id,
    commit_count: 12,
    repository_size: usage * 3 / 5,
    lfs_objects_size: usage * 2 / 5
  )

  project_statistics.save!
end

root_user = User.where(username: 'root').first

create_group_with_usage_and_additional_storage(root_user, 'over-free-limit', 0, 20.gigabytes)
create_group_with_usage_and_additional_storage(root_user, 'under-free-limit-no-purchase', 0, 8.gigabytes)
create_group_with_usage_and_additional_storage(root_user, 'over-purchased-limit', 1, 30.gigabytes)
create_group_with_usage_and_additional_storage(root_user, 'warning-with-purchase', 1, 17.gigabytes)
create_group_with_usage_and_additional_storage(root_user, 'under-free-limit-with-purchase', 1, 5.gigabytes)

Go to http://localhost:3000/groups/over-purchased-limit/-/usage_quotas#storage-quota-tab

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