Move `seats usage` from billing to usage quotas

What does this MR do?

Closes #334861 (closed)

As part of the long-term vision for Seat Usage documented in &5872 we will move the Seat Usage page from the current location within Group settings > Billing directory, to Group settings > Usage Quotas.

Screenshots or Screencasts (strongly suggested)

Before	After

How to setup and validate locally (strongly suggested)

You need to have a license on your GitLab instance locally in order to enable these views.

Migration of the UI

1. Go to Menu > Groups > Your Group
2. Chose Settings > Usage Quotas
3. You'll see the Seats usage UI, with the billable users in "Your Group"
4. The URL should be /groups/<your group>/-/usage_quotas#seats-quota-tab

Back end redirection

1. If you place this path on your browser: /groups/<your group>/-/seat_usage
2. You should be redirected to /groups/<your group>/-/usage_quotas#seats-quota-tab

Migration of the link

1. Go to Menu > Groups > Your Group
2. Chose Settings > Billing
3. Under "Seats currently in use" click in the button Seat usage
4. You should be redirected to /groups/<your group>/-/usage_quotas#seats-quota-tab

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
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• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.