Add field for curator's comments in digitization section

[travs]

Maybe we could add this to all sections, but it seems like a place for unstructured comments about the data or difficulties in the digitization process might be helpful.

[VahidGh]

This is considered for the experiment, and as the whole curation process is available through one page, we can use the same field, for any related comment.

The thing we need is to be able to view/edit the curation for each experiment/channel the same as the creation process. @miladjafary