chore: use console email backend in debug

chore: edit training reminder text and time

feat: add CheckoutHistory model to track checkin deletions

Record checkout reason and timestamps when a student cancels,
a trainer removes a checkin, a training is deleted, or its
time changes.

Closes #401

feat: replace native confirm with SweetAlert2 for student deletion

chore: allow to cancel checkin by trainer only during training editab…

…le interval

fix: do not duplicate filter keys in url

fix: training start and end should have date subfield

chore: allow staff or trainers to get info about training

feat: support college

feat: allow to upload multiple medical group reference images

Related to #323