[bug] Reviewers can be added multiple times with the same email address

[cosenal]

Small bug: the form under «Review team members» on the Devroom-dashboard page doesn't check for duplicates, so one can add the same email address multiple times. I suspect that in that case multiple invitations are also sent to the same address, although I haven't checked that.

This is also a feature request about devroom managers being able to remove reviewers from the same form, or to cancel a pending invitation, which doesn't seem possible at the moment.