Allow pgcrypto extension to preexist (#1114)

The CREATE EXTENSION query requires superuser privileges, which means
that the Firezone container may not be allowed to execute it when
accessing an externally hosted database, resulting in the database
failing to initialise properly.

One potential workaround to this problem is to have the database admin
ascertain that the pgcrypto extension is already present and ready for
use, prior to starting the Firezone container for the first time.

Unfortunately, however, this currently fails in the following manner:

14:42:44.731 [info] execute "CREATE EXTENSION pgcrypto"
** (Postgrex.Error) ERROR 42710 (duplicate_object) extension "pgcrypto" already exists

Adding the IF NOT EXISTS condition to the CREATE EXTENSION query should
allow it to succeed.

apps/fz_http/priv/repo/migrations/20220124162404_add_uuids.exs

@@ -3,7 +3,7 @@ defmodule FzHttp.Repo.Migrations.AddUuids do
 
   def change do
     execute(
-      "CREATE EXTENSION pgcrypto",
+      "CREATE EXTENSION IF NOT EXISTS pgcrypto",
       "DROP EXTENSION pgcrypto"
     )