Upgrade Peewee to latest 3.x

This requires a number of small changes in the data model code, as well as additional testing.

data/model/test/test_basequery.py

@@ -1,6 +1,6 @@
 import pytest
 
-from peewee import JOIN_LEFT_OUTER
+from peewee import JOIN
 from playhouse.test_utils import assert_query_count
 
 from data.database import Repository, RepositoryPermission, TeamMember, Namespace
@@ -87,7 +87,7 @@ def test_filter_repositories(username, include_public, filter_to_namespace, repo
            .distinct()
            .join(Namespace, on=(Repository.namespace_user == Namespace.id))
            .switch(Repository)
-           .join(RepositoryPermission, JOIN_LEFT_OUTER))
+           .join(RepositoryPermission, JOIN.LEFT_OUTER))
 
   with assert_query_count(1):
     found = list(filter_to_repos_for_user(query, user.id,