Refactor sightings data model adding sighting's jellyfishes size and abundance as separate models.

Before this commit, jellyfishes where created as variables (presence, size and abundance), and related to sightings among with an certain value.

Now Jellyfish is a model by itself, as the possible values of size and abundance. A sighting (Sight model) is related to several jellyfishes, among with a FK to their size and abundance.
Add jellyfishes sizes and abundances to sights app initial data. Drop FTP as reporting client.
Add jellyfishes presence field and jellyfishes fieldset to SightForm
Make jellyfishe presence checkbox toggle jellyfishes fieldset visibility
Add jellyfishes to sights' app initial data, among minor changes in model verbose names and form labels.
Approach to dinamically generate jellyfishes inputs in new sight form using mustache
Approach to management command importing sightings from proteccion civil's service
Add migration renaming beach's api_id to medjelly_api_id
Merge branch 'refactor_data_model'