Fix words endpoint for protected words

[ztefanie]

Describe the Bug

Requesting https://lunes-test.tuerantuer.org/api/words/1648 with correct api-key returns 404 Not Found.
Requesting https://lunes-test.tuerantuer.org/api/documents/248 with same api-key returns the word.

Expected Behavior

/words/:id-of-word should return the word also for protected vocabulary.

Actual Behavior

/words/:id-of-word returns 404 for protected vocabulary.

Additional Information

We need this feature for the app, as users can save vocabulary as favorites. We only save the id's and api-keys for favorites, so we cannot use the documents-endpoint, as we do not know the discipline-id of the word in this context.

[timobrembeck]

I think this was a known caveat when implementing the feature, see #304.
Nevertheless it should be possible to add support for protected words without too much effort.