1571 calendar improvements No. 3

This is a MR for item No 3 in issue #1571. It adds a "Mass Select/Deselect" function in both Shared and My Calendar Views. The background colour of the mass select widget is hardcoded to match the dark grey used for the left menu and calendar heading.

This MR comprises two commits. One to modify the templates and language file, and the other to add the Javascript event handler.

Status changed to merged

mentioned in commit 91146beb

@lord_alan Thanks! for the merge request, It's been pulled to 7.4.0