Group Deploy Keys - preparatory work

Summary

This issue is to be addressed in preparation of the Group Deploy Keys development. It will include:

Refactoring parts of deploy keys frontend and backend so that they can be used both for project-scoped and group-scoped deploy keys. MR: !30225 (merged)
~~add a deploy_key_typeto thekeys` table. MR: !30240 (closed)~~ not doing this in the end. Doing the next item instead.
database: create the group_deploy_keys table. MR: !30886 (merged)
database: add a deploy_keys_groups intermediate table. MR in progress: !32901 (merged)

Edited Jun 15, 2020 by Etienne Baqué