Restrict users from canceling another user's push requests

[sam-holmes2]

Is your feature request related to a problem? Please describe.
Currently a user could cancel another user's push requests - potentially disrupting CI

Describe the solution you'd like
Additional authorization should be implemented to ensure users can only cancel their own push requests

[kriswest]

n.b. They can only do if they are also authorized to push on that particular project I think.

We should preserve the ability for an admin user to cancel if/when necessary.

[jescalada]

@sam-holmes2 Do you mean any unprivileged user can cancel another user's PR, as long as they're in the authorisedList?

We should preserve the ability for an admin user to cancel if/when necessary.

Seconding this. Perhaps only admins should be allowed to cancel any PR.