We have to run python manage.py migrate once, in order to create the necessary tables for the postgres search backend.
Otherwise we get an error, as the insert in the table is not possible.
* Adds PostgreSQL search backend.
* Isort nitpicks.
* Fixes PostgreSQL versions incompatibilities.
* Uses Django lru_cache instead of building our own.
* Fixes PostgreSQL search index on some empty vector & query cases.
* Never sets the PostgreSQL search vector to NULL.
* Simplification + removes caching on two fast enough functions.
* Rewrites stale entries deletion to use the ORM.