Declare bootstrap as a peer dependency

What does this MR do?

Declares bootstrap as a peer-dependency. Now that gitlab-ui declares its dependency on bootstrap, gitlab-ce imports bootstrap-styles through gitlab-ui, and we aim to depend on gitlab-ui for the UI components used in the gitlab application, moving bootstrap to the peerDependencies will help us to avoid conflicting versions of bootstrap between the gitlab-ui and gitlab-ce.

Related merge requests:

• gitlab-ui!204 (merged)
• https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/26165