Implement /admin using flask-admin. Overall cleanup

app/extensions/nopaque_flask_admin_views.py

@@ -0,0 +1,20 @@
+from flask import abort
+from flask_admin import (
+    AdminIndexView as _AdminIndexView,
+    expose
+)
+from flask_admin.contrib.sqla import ModelView as _ModelView
+from flask_login import current_user
+
+
+class AdminIndexView(_AdminIndexView):
+    @expose('/')
+    def index(self):
+        if not current_user.is_administrator:
+            abort(403)
+        return super().index()
+
+
+class ModelView(_ModelView):
+    def is_accessible(self):
+        return current_user.is_administrator

app/extensions/nopaque_sqlalchemy_extras/__init__.py

@@ -1,2 +0,0 @@
-from .types import ContainerColumn
-from .types import IntEnumColumn