Add cancel button and fix validation bug on Get Involved form

[elichad]

Form was raising a server error if submitted blank, because the validation on trainee_notes was expecting involvement_type to be set.
Note that involvement_type is always required in TrainingProgress creation, but it has required=False set on the form. This is because at least one other field will always be required, but this is only determined at validation time, so showing an asterisk on only one field and not the rest doesn't quite represent how the form works.

[pbanaszkiewicz]

LGTM