Before this change, the THIRDPARTY_EXTENSIONS_PATH would end up with a
double-slash in the path, which was breaking FreshRSS's is_valid_path
detection.
This avoids restarting the postgresql server, when only ensureDatabases
or ensureUsers have been changed. It will also allow to properly wait
for recovery to finish later.
To wait for "postgresql is ready" in other services, we now provide a
postgresql.target.
Resolves#400018
Co-authored-by: Marcel <me@m4rc3l.de>