The added context variables seem to be ununused. Remove the method.

We are going to feed internal and external events as separat event
sources to the FullCalendar. It is more efficient to specify the color
for the whole event source. So remove it from what get_dict returns for
an event.

Retrive internal and external separately and use them as event sources
for FullCalendar.

Provide button to switch of and back on the "external" event source.

Make the label of the button with toggles display of extern events
translatable. Put the two translated strings into template variables,
because one of it is used multiple times in the template.

Create django.po with

    ./manage.py makemessages -l de --ignore bla/lib/python3.7/site-packages/xlwt