Maintenance: Create DistrictConfigLog to enable more self-serve district maintenance

[kevinrobinson]

For now, this is moving some district configuration that district IT folks provide out of config code and into the database, where it could be stored, viewed and edited in a more self-serve way.

Concretely, that's done by moving where PerDistrict reads from into DistrictConfigLog. It typically expresses differences directly in code, reads from ENV variables, or reads config from files like district_somerville.yml.

The idea is that PerDistrict reads secrets and info about services from ENV that's controlled by developers, but that other information (eg, URLs for sheets or drive folders being synced, safelists for particular features, etc) should be read from DistrictConfigLog where it's a step closer to being exposed to project leads and district IT staff.

[kevinrobinson]

selfie