zcommands: Add zcommand module and separate test module.

Move the zcommands from '/views/messages.py' to
'/lib/zcommand'.

Also, move the zcommand tests from '/tests/test_messages.py'
to '/tests/test_zcommand'.

zerver/lib/zcommand.py

@@ -0,0 +1,32 @@
+from typing import Any, Dict
+from django.utils.translation import ugettext as _
+
+from zerver.models import UserProfile
+from zerver.lib.actions import do_set_user_display_setting
+from zerver.lib.exceptions import JsonableError
+
+def process_zcommands(command: str, user_profile: UserProfile) -> Dict[str, Any]:
+
+    if command == 'ping':
+        ret = dict()  # type: Dict[str, Any]
+        return ret
+
+    if command == 'night':
+        if user_profile.night_mode:
+            msg = 'You are still in night mode.'
+        else:
+            msg = 'Changed to night mode! To revert night mode, type `/day`.'
+            do_set_user_display_setting(user_profile, 'night_mode', True)
+        ret = dict(msg=msg)
+        return ret
+
+    if command == 'day':
+        if user_profile.night_mode:
+            msg = 'Changed to day mode! To revert day mode, type `/night`.'
+            do_set_user_display_setting(user_profile, 'night_mode', False)
+        else:
+            msg = 'You are still in day mode.'
+        ret = dict(msg=msg)
+        return ret
+
+    raise JsonableError(_('No such command: %s') % (command,))