Add migration to Sighting model so jellyfish info non-mandatory
Empties jellfish data in sight report form when "other specimen" choosen
Rename description_extra Sighting's attribute to other_specimen_description.
Add corresponnding migration.
Migration extending number of integer digits in longitude field
Migration adding specimen_type attribute to Sightings. Refactor so it works in sight report form.
Move sight report form's default jellyfish image selection to JS logic
Merge branch 'jellyfish_data_not_required' into development