Ebird-checklists

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* Added Checklist and Observation Managers for convenience queries.

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* Split the load_csv management command into separate commands for loading eBird
Basic Dataset, load_dataset, and MyEBirdData, load_mydata.

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* Added Checklist and Observation comments - newly available from the eBird API.

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* Removed rogue import, ``from tomlkit import datetime`` - thanks PyCharm.

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* Changed the APILoader again (sorry), renaming the modes to 'new' and 'all'.
* Removed update_checklists() and update_checklist(). Instead load_checklists()
was refactored to handle both cases, by adding ``new_only`` as an argument.

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* Added methods update_checklists() and update_checklist() to the APILoader
for updating existing checklists.
* Added Checklist.started to record the date and time the checklist was started.
This is just the compound value of Checklist.date and Checklist.time.
* Added indexes to model fields.
* Added translation strings.
* Changed APILoader.get_location() so it always returns a valid Location.
Previously, if a location did not exist it created a placeholder with only the
identifier set. It now uses the ebird-api's new get_location() method to always
returns a valid Location.
* Changed the APILoader to add two modes: add-checklist and update-checklists. The
add-checklists mode only adds new checklists - greatly reducing the load on the
eBird servers. The update-checklists mode updates all the checklists for a given
date. Generally, less than 1% of checklists are updated, so avoid using this mode
unless it's really necessary. However in this case you should consider using the
eBird Basic Dataset instead.

.. code-block:: console

Load new checklists submitted in the past 7 days
python manage.py load_api add-checklists 7 US-NY

Update checklists submitted in the past 3 days
python manage.py load_api update-checklists 3