Optimise Collaborators forms and views

[data-sync-user]

We should optimise the subscriber/unsubsciber form to use the parent and mixin concept to avoid duplicate code like we do here example https://github.com/mozilla/experimenter/blob/6bb82d44bae92a6d92d5d5cdc14dadcbec62f4e8/experimenter/experimenter/nimbus_ui/views.py#L612 and here example https://github.com/mozilla/experimenter/blob/6bb82d44bae92a6d92d5d5cdc14dadcbec62f4e8/experimenter/experimenter/nimbus_ui/forms.py#L1258

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