WIP: Add namespace column to user_callouts

What does this MR do?

Add namespace column to user_callouts table.

To be used in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9809

As explained in https://gitlab.com/gitlab-org/gitlab-ee/issues/8237#note_146586920, we would like to have per-namespace user callouts. For example, a callout of type TANUKI_CALLOUT dismissed on gitlab-org should not be dismissed on not-gitlab-org.

Post-deployment data migration

If this is deployed to production after the initial version of https://gitlab.com/gitlab-org/gitlab-ee/issues/8237 lands, we will have a some GOLD_TRIAL_BILLINGS callout rows that wont have a namespace value and should be deleted as this callout will be ignored and those rows will sit around doing nothing. I don't expect there to be lots but it's going to be worth cleaning up so we have more consistent and predictable records.

If this is merged before then and we can merge an MR that updates the first version before its even deployed, we wont need to do a data migration. Chances seem slim given its the 2nd.

What are the relevant issue numbers?

Part of https://gitlab.com/gitlab-org/gitlab-ee/issues/8237

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