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>

website/common/script/fns/getUtcOffset.js

@@ -0,0 +1,12 @@
+/**
+ * Converts from timezoneOffset (which corresponded to the value returned
+ * from moment.js's deprecated `moment#zone` method) to timezoneUtcOffset.
+ *
+ * This is done with conversion instead of changing it in the database to
+ * be backwards compatible with the database values and old clients.
+ *
+ * Not as a user method since it needs to work in the frontend as well.
+ */
+export default function getUtcOffset (user) {
+  return -(user.preferences.timezoneOffset || 0);
+}