Fix#820
Ok, so after I would have to use the same method in three different
locations (`<ds-card>` expects an `image` attribute but cannot render
entire components) I decided to implement the prefix of image urls with
a filter rather than a component. The downside of this is that we have
to add the filter on a lot of component tests. The benefit is less
components and hopefully less complexity.
The crucial test is still missing: Write a test to check that no more
than one loading spinner is visible all the time.
@Tirokk FYI I wrote a page component test and made sure that the test
will also fail if the implementation is broken.
