Set current organization when adding new deploy keys

What does this MR do and why?

Sets the current organization when adding new deploy keys (via API or UI).

References

#577244

How to set up and validate locally

1. Go to Admin -> Deploy keys and add a new deploy key. In the Rails console, verify the organization was set correctly (DeployKey.last.organization).
2. Go to any project's Settings -> Repository -> Deploy keys and perform the same check.
3. Perform the same check after creating a key via the project and admin deploy key APIs.

MR acceptance checklist

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