Set bot_namespace when creating project_bot user

What does this MR do and why?

Part 3 of Credentials inventory project and group access ... (#498333 - closed)

Now that we have a bot_namespace_id column in user_details(once !170309 (merged) is merged) start populating that column for new project_bot users (project and group access tokens). This will later be used to query for users and tokens to display in the GitLab.com credentials inventory.

This MR is dependent upon !170309 (merged) being merged, but that is in final review. To keep things moving we can do initial review on this MR.

References

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• Credentials inventory project and group access ... (#498333 - closed)
• Add bot_namespace column and foreign key to use... (!170309 - merged)

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