Python 3.8 is minimal version now

Close #49

Added requirements and installation instructions for using PostgreSQL and apparently there were another migration necessary.

* for adding a number of (business) days to the schedule (e.g. "on the rrules' following day")
* fixed a bug for monthly recurrences
* and some minor changes

See #8

and:
* Set correct login redirect url for oidc
* Added django-cors-headers for cross-domain api requests

See #21 #22

See #22

* Properly generate thumbnails for shows, hosts and notes
* Return thumbnails in API

See #22

...as well as some minor changes and bug fixes.

See #8 #10 #22 #24

"syncdb" is deprecated: "migrate" is the command to create the DB.