Fix end date for Google Calendar

Use Liquid filter to increase the date by one day.

I tested this and Google is smart to know
that YYYY0132, i.e. January 31st + 1, is YYYY0201,
i.e. February 1st.

Close #384.

_includes/workshop_calendar.html

@@ -0,0 +1 @@
+<a href="//calendar.google.com/calendar/render?action=TEMPLATE&text={% if page.carpentry == "swc" %}Software{% elsif page.carpentry == "dc" %}Data{% endif %} Carpentry Workshop&dates={{ page.startdate | replace: "-", "" }}/{{ page.enddate | replace: "-", "" | plus: 0}}&trp=false&sprop&sprop=name:&sf=true&output=xml&location={{ page.address }}&details={% if page.carpentry == "swc" %}Software{% elsif page.carpentry == "dc" %}Data{% endif %} Carpentry Workshop at {{ page.venue }}">Add to your Google Calendar.</a>

index.html

@@ -120,7 +120,7 @@ and the administrator will contact you if we need any extra information.</h4>
 <p id="when">
   <strong>When:</strong>
   {{page.humandate}}.
-  <a href="//calendar.google.com/calendar/render?action=TEMPLATE&text={% if page.carpentry == "swc" %}Software{% elsif page.carpentry == "dc" %}Data{% endif %} Carpentry Workshop&dates={{ page.startdate | replace: "-", "" }}/{{ page.enddate | replace: "-", "" }}&trp=false&sprop&sprop=name:&sf=true&output=xml&location={{ page.address }}&details={% if page.carpentry == "swc" %}Software{% elsif page.carpentry == "dc" %}Data{% endif %} Carpentry Workshop at {{ page.venue }}">Add to your Google Calendar.</a>
+  {% include workshop_calendar.html %}
 </p>
 {% endif %}