* 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.
