docs: Simplify SQL for user management how-tos

All those DROP IF EXISTS and CREATE IF NOT EXISTS etc. just give a lot more text to
read and understand. If in fact a user creates the same thing twice, they should be
able to understand the error message from postgres.

docs/how-tos/sql-user-management-using-postgres-users-and-passwords.rst

@@ -31,7 +31,7 @@ As in :ref:`sql_user_management`, we create a :code:`basic_auth` schema:
   -- We put things inside the basic_auth schema to hide
   -- them from public view. Certain public procs/views will
   -- refer to helpers and tables inside.
-  CREATE SCHEMA IF NOT EXISTS basic_auth;
+  CREATE SCHEMA basic_auth;
 
 
 As in :ref:`sql_user_management`, we create the :code:`pgcrypto` and :code:`pgjwt` extensions. Here we prefer to put the extensions in its own schemas:
@@ -40,7 +40,7 @@ As in :ref:`sql_user_management`, we create the :code:`pgcrypto` and :code:`pgjw
 
   CREATE SCHEMA ext_pgcrypto;
   ALTER SCHEMA ext_pgcrypto OWNER TO postgres;
-  CREATE EXTENSION IF NOT EXISTS pgcrypto WITH SCHEMA ext_pgcrypto;
+  CREATE EXTENSION pgcrypto WITH SCHEMA ext_pgcrypto;
 
 
 Concerning the `pgjwt extension <https://github.com/michelp/pgjwt>`_, please cf. to :ref:`client_auth`.
@@ -49,7 +49,7 @@ Concerning the `pgjwt extension <https://github.com/michelp/pgjwt>`_, please cf.
 
   CREATE SCHEMA ext_pgjwt;
   ALTER SCHEMA ext_pgjwt OWNER TO postgres;
-  CREATE EXTENSION IF NOT EXISTS pgjwt WITH SCHEMA ext_pgjwt;
+  CREATE EXTENSION pgjwt WITH SCHEMA ext_pgjwt;
 
 
 In order to be able to work with postgres' SCRAM-SHA-256 password hashes, we also need the PBKDF2 key derivation function. Luckily there is `a PL/pgSQL implementation on stackoverflow <https://stackoverflow.com/a/72805848>`_:
@@ -117,7 +117,7 @@ In order to be able to work with postgres' SCRAM-SHA-256 password hashes, we als
   ALTER FUNCTION basic_auth.pbkdf2(salt bytea, pw text, count integer, desired_length integer, algorithm text) OWNER TO postgres;
 
 
-Analogous to :ref:`sql_user_management` creates the function :code:`basic_auth.user_role`, we create a helper function to check the user's password, here with another name and signature (since we want the username, not an email address).
+Analogous to how :ref:`sql_user_management` creates the function :code:`basic_auth.user_role`, we create a helper function to check the user's password, here with another name and signature (since we want the username, not an email address).
 But contrary to :ref:`sql_user_management`, this function does not use a dedicated :code:`users` table with passwords, but instead utilizes the built-in table `pg_catalog.pg_authid <https://www.postgresql.org/docs/current/catalog-pg-authid.html>`_:
 
 .. code-block:: postgres