Remove the thumbnail field from the corpus edition form

Found while working on backend#1101 (closed).

Corpus.thumbnail is a foreign key pointing to an element that must have an image and that should be used as the project's thumbnail. This was done for the %Projects Introduction milestone, which was meant to add a more complex Markdown description to make projects like IFAO browsable by the general public.

This milestone was abandoned, but only after the thumbnail foreign key got added in the backend (backend#698 (closed)) and made editable in the frontend (#615 (closed)). The actual display of the thumbnail (#617 (closed) and #618 (closed)) never got done, so this thumbnail is unnecessary. There are possible bugs in the current implementation, since nothing stops someone from removing the image of an element used as the thumbnail for example—and I found that through deductions by reading the code, not by actually using the feature in the frontend.

We should either actually implement thumbnails, or drop them completely, but not leave this feature in the technical debt limbo…