add a description field to the file model

To simplify the upload of files, we should add the description field to the File model and accept it during the creation of a file.

changed milestone to %1.0-alpha7 — Helpful Hedgehog

assigned to @eigenwijsje

marked this issue as related to #86 (closed)

mentioned in issue #86 (closed)

added P1 label

added size:s label

added To Do label

added Cycle [2/25] label