Move from deprecated moment#zone to moment#utcOffset (#12207)

* Issue 10209 - Remove read usages of zone

* Issue 10209 - Add coverage on daysSince and startOfDay cron utility functions

* Issue 10209 - Add unit test for daysUserHasMissed method

* Issue 10209 - Remove usages of deprecated `moment.js#zone` method.

* Issue 10209 - Add helper function to centralise logic

Also simplify timezoneOffsetToUtc function in site.vue

* Issue 10209 - Also add getUtcOffset as method on user

Co-authored-by: Matteo Pagliazzi <matteopagliazzi@gmail.com>

test/api/unit/libs/preening.test.js

@@ -9,7 +9,7 @@ describe('preenHistory', () => {
   beforeEach(() => {
     // Replace system clocks so we can get predictable results
     clock = sinon.useFakeTimers({
-      now: Number(moment('2013-10-20').zone(0).startOf('day').toDate()),
+      now: Number(moment('2013-10-20').utcOffset(0).startOf('day').toDate()),
       toFake: ['Date'],
     });
   });