Calendar plugin and associated events feature

[jcfr (Jean Christophe Fillion Robin (Kitware))]

I experimented with the now available calendar plugin and also enabled the experimental events feature:

• This post includes an event
• The event is now listed in a calendar displayed at the beginning of the category

cc: @Sam_Horvath @lassoan @pieper

[lassoan (Andras Lasso)]

This event feature looks nice. The calendar in the category main page is useful, too (perhaps a bit too big).

[RafaelPalomar]

This is very useful. The event shows up in local time