logging: Make our own Formatter class.

This doesn't yet do much, but it gives us a suitable place to
add code to customize how log messages are displayed, beyond what
a format string passed to the default formatter can do.

tools/linter_lib/custom_check.py

@@ -204,8 +204,6 @@ def build_custom_checkers(by_lang):
          'exclude_line': set([
              ('zerver/views/users.py',
               "return json_error(_(\"Email '%(email)s' not allowed for realm '%(realm)s'\") %"),
-             ('zproject/settings.py',
-              "'format': '%(asctime)s %(levelname)-8s %(message)s'"),
              ('static/templates/settings/bot-settings.handlebars',
               "'https://hostname.example.com/bots/followup'"),
          ]),