ro_user: grant execute only on defined functions

GRANT EXECUTE ON ALL FUNCTIONS in SCHEMA public is causing an error on prod:

 Traceback (most recent call last):
   File "/home/appuser/app/src/AbstractHandler.py", line 101, in on_message
     self.act(content, message)
   File "/home/appuser/app/src/CreateThingInPostgresHandler.py", line 59, in act
     self.grant_sta_select(thing, user_prefix=sta_prefix)
   File "/home/appuser/app/src/CreateThingInPostgresHandler.py", line 227, in grant_sta_select
     c.execute(
 psycopg2.errors.InsufficientPrivilege: permission denied for function pg_stat_statements_reset

the grant on execution on all functions in public should be removed and replaced by only the functions from postgis that are needed (e.g. ST_AsGeoJSON, ...)