Extend deploy tokens to multiple projects

Description

Deploy tokens give read-only access to a project. Sometimes users need to share the same access to multiple projects.

Users should be able to allow deploy tokens in other projects too, like we have for deploy keys.

Possible alternative/improvement to: https://gitlab.com/gitlab-org/gitlab-ce/issues/45066

Proposal

When adding a deploy token, allow Masters to add an existing token from another project they have access too.

This brings a few security concerns:

1. should value be shown when adding?
2. which are the access level in the source and target project to allow "importing"?
3. what if a token is revoked in one of the projects?