Add controllers for user contribution mapping

What does this MR do and why?

This MR implements the assignee-facing controllers described in #443555 (closed).

The proposed flow is that a user receives an email asking for permission to attribute imported contributions within a namespace to them. The email would include an accept/reject link, directing the (signed in) user to one of two controller actions.

The controller then updates the appropriate Import::SourceUser record according to whether the user accepted or rejected the attribution.

This MR is intentionally a partial solution to the linked issue. It does not include any of the group admin facing GraphQL endpoints. This is both to keep this MR manageable, and to reduce the number of cycles needed to make the :improved_user_mapping flag available to the rest of the team working on this feature.

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