Add support for downloading report in readable format

[petterill]

Support for ready-made hearing reports in a format such as txt or word or pdf.

[Rikuoja]

Oh, I deleted the frontend issue City-of-Helsinki/kerrokantasi-ui#559 as basically specifying should be done here. So my comments there are as follows:

Currently, the data in the excel format is specific fields directly from the database. It is possible to create a pdf, text or word renderer for a specific endpoint in the API, but it would require specifying user needs for such a report: should all comments be included? Should comment images be included? How do we format it?

Do we make some analysis and processing before creating the report? Without that the pdf format is not any better than excel in my opinion.