Add group_deploy_keys_groups intermediate table

What does this MR do?

This MR adds the group_deploy_key_groups intermediate table, connecting the groups and the group_deploy_keys tables. This is the last MR that finishes some preparatory work, before we take care of the main development of Group Deploy Key.

Manual testing

etienne@dell ~/src/gdk/gitlab(215160-add-intermediate-group-deploy-key-table ✗) rails db:migrate:up VERSION=20200522205606
== 20200522205606 CreateGroupDeployKeysGroup: migrating =======================
-- create_table(:group_deploy_keys_groups)
   -> 0.0266s
== 20200522205606 CreateGroupDeployKeysGroup: migrated (0.0267s) ==============

etienne@dell ~/src/gdk/gitlab(215160-add-intermediate-group-deploy-key-table ✗) rails db:migrate:down VERSION=20200522205606
== 20200522205606 CreateGroupDeployKeysGroup: reverting =======================
-- drop_table(:group_deploy_keys_groups)
   -> 0.0016s                                                 
== 20200522205606 CreateGroupDeployKeysGroup: reverted (0.0306s) ==============

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Closes #215160 (closed)