Aestimia integration

This introduces a few new environment variables (and changes one):

- `CSOL_HOST` now needs to be a fully canonical URL, e.g. `http://chicagosummeroflearning.org`. It used to be just a hostname, for email purposes.
- `CSOL_EMAIL_DOMAIN` is an optional variable, that only needs to be set if the email domain is different to the host (and what was `CSOL_HOST`).
- `CSOL_AESTIMIA_URL` is the location of the Aestimia instance.
- `CSOL_AESTIMIA_SECRET` is the secret shared with Aestimia.