Make start date and day of month aware of timezones (fixes #12555) (#12696)

* WIP: #12555

-dayOfMonth takes timezone into account on task update for monthly dailys
-startDate set to start of day in user's tz on task update for monthly
dailys
-tweaked a test so that it's actually testing something

* WIP: 12555

-task.startDate gets set to start of day in user's tz on task creation
and task update
-set preferences.timezoneOffset to nonzero value in certian test user objects
-removed date literals in test:api-v3:integration:tasks:PUT

test/api/v3/integration/tasks/challenges/PUT-tasks_challenge_challengeId.test.js

@@ -12,7 +12,7 @@ describe('PUT /tasks/:id', () => {
   let challenge;
 
   before(async () => {
-    user = await generateUser();
+    user = await generateUser({ 'preferences.timezoneOffset': new Date().getTimezoneOffset() });
     guild = await generateGroup(user);
     challenge = await generateChallenge(user, guild);
     await user.post(`/challenges/${challenge._id}/join`);