The "run_validators" method of rest_framework.serializers.Serializer
seems to expect a dict. Thus deserialization ("to_internal_value")
fails, since it returns a proper "Gestalt" instance (which cannot be
handled by "run_validators").

Fix a bug in the calendar export code, that breaks for events without an
end time ("until_time").

It is possible to specify events without an end.
These events currently trigger an exception during a calendar export.

* reduce a bit of code duplication
* use unique content for each test
* fix wrong order of dictionary overrides:
  "kwargs.update(...)" -> "...update(kwargs)"
  The problem was not visible, since only "public" was overriden (which
  was not part of the default dictionary).

* replace fa icon references with semantic sg descriptors
* only include necessary parts of font-awesome
* add missing size modifiers