[en-GB] Remove leading space from date in calendar menu

In the date button at the top of the right column of the calendar menu, on days 1 to 9 of the month, there's a leading space before the date, for example “ 4 May 2020”.

This makes the date appear misaligned versus the weekday above. (It's more noticeable when the date isn't the 4th.)

Change the date format to remove the leading space.