- Ensure that the doer and blocked user cannot add each other as
collaborators to repositories.
- The Web UI gets an detailed message of the specific situation, the API
gets an generic Forbidden code.
- Unit tests has been added.
- Integration testing for Web and API has been added.
- This commit doesn't introduce removing each other as collaborators on
the block action, due to the complexity of database calls that needs to
be figured out. That deserves its own commit and test code.
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