Avoid date inconsistency on due date picker

There is a date inconsistency when we instantiate the Date object passing the date as string and we should avoid it as the MDN recommends - https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/Date

This inconsistency is the reason for the bug described here: https://gitlab.com/gitlab-org/gitlab-ce/issues/24253.

Below, we can see different results according to the use of the string. For example, with dashes, the browser (Chrome) ignores the time zone which results in a wrong date. In order to solve this, we can instantiate the Date object passing the year, month (zeroBased) and day as integer params.

Closes #24253 (closed)