feat: add problem checks + handler feature

the basic idea is to run nightly checks and send email if problems are
found.  it should also support variations on that theme,
e.g. configuring a check to only run on certain weekdays.

tests/test_app.py

@@ -634,6 +634,12 @@ app_title = WuttaTest
         people = self.app.get_people_handler()
         self.assertIsInstance(people, PeopleHandler)
 
+    def test_get_problem_handler(self):
+        from wuttjamaican.problems import ProblemHandler
+
+        handler = self.app.get_problem_handler()
+        self.assertIsInstance(handler, ProblemHandler)
+
     def test_get_report_handler(self):
         from wuttjamaican.reports import ReportHandler