Multiple bugfixes for instructor calendar; improve scheduler human insight

Fix off-by-one retrieval bug in new instructor calendar
Refactor scheduler/solver to display which dates are excluded for which classes before the solver is run, so the instructor knows what's going on
Pull out base Airtable methods into airtable_base
Add a couple methods for future logging of sign-in submissions via Artable (migrate off Google sheets) and start of support for survey submission from welcome page

protohaven / protohaven_api