Calendar arrows need aria-hidden="true"

In these three slightly different uses of larrow and rarrow in calendar templates, the arrow should be marked as aria-hidden so that it isn't read out by screen readers.

calendar/templates/month_navigation.mustache
38: }} class="arrow_link previous"{{!
53: }} class="arrow_link next"{{!

calendar/templates/month_mini.mustache
50: }} class="arrow_link previous"{{!
64: }} class="arrow_link next"{{!

calendar/templates/day_navigation.mustache
38: }} class="arrow_link previous"{{!
53: }} class="arrow_link next"{{!

In these specific examples, the browser will then pick up the title text to use as the link text e.g. "next month" see screenshots attached from Firefox developer tools.