Add a designs model

What does this MR do?

Add the database model for Design management:

The Design model represents a single file in, it will be linked to an issue and can have many Design::Versions. The discussions would then take place on that version.

Keeping track of the Version in a separate table allows us to easily keep track of the last commit that touched a file without having to call out to Gitaly.

I did not add an overarching model that has many designs, but instead referenced the the project and issue directly on the design model. I don't think there's any added value in having the extra table.

What are the relevant issue numbers?

First step in https://gitlab.com/gitlab-org/gitlab-ee/issues/9492

Does this MR meet the acceptance criteria?

• [-] Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?
• Security reports checked/validated by reviewer