Move reusable Danger files to a separate Gem

Currently CE and EE have a bunch of custom Danger checks, many of which can be reused across different projects we have. For example, the checks in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/21527 are not specific to CE and could be useful for other projects, such as Gitaly, Omnibus, etc.

We should identify the Danger files we can reuse (basically all that are not specific to CE and EE), move those to a Gem as Danger plugins.