Item description is now filled if it is a sample and cannot be changed in this case

Fixes #311 (closed).

If the item is a sample, the description is filled from the SMIS sample (empty in this case) and cannot be edited by the user.

The description can still be edited for tool or other:

In regard of the discussion in #311 (closed), @demariaa and I decided to keep the comments field for tool and other but I changed the description field to a single-line (rather than a text block).

This way, the user gets the feeling that description should be succinct and that long texts should be put in comments.

Also fixes #330 (closed) and !318 (comment 82863).