Scale canary deployments

Description

With https://gitlab.com/gitlab-org/gitlab-ce/issues/38543 you can scale your deployments using the CI/CD > Environments page. If you use canary deployments, you should be able to do the same.

Proposal

Implement a flow to scale canary deployments from the CI/CD > Environments page.

Design

The design is from https://gitlab.com/gitlab-org/gitlab-ee/issues/5416

The labels and all the copying should be revisited to adapt to this new scope.

(TODO: Where is the documentation links to?)

Tooltips

It has the short help text in a tooltip to explain the terms of Instances and Incremental rollouts.

(Note: the copy is not ready yet.)