Dates as Python objects

[ntarocco]

• date and datetime fields are now converted to Python datetime objects before executing transitions and converted back to ISO format when storing the loan record
• add request_start_date to be able to track the start and in the end of a PENDING loan (request)
• moves transitions decorators checks from before method to execute to ensure decorators are executed as first thing
• makes configurable to automatically assign the item when creating a request via assign_item param in the transitions
• adds marshmallow loader for REST endpoint
• adds support to ES7
• changes permissions to take advantage of invenio-access
• ES mappings are now matching schema fields
• Drops support for Python 2
• closes #111, #106, #76, #81

To reviewers: I apologize for the big PR, but I had to change many things after the introduction of the REST loader with fields validation. Bonus are ES 7 support plus Python 2 drop.

[ntarocco]

@BadrAly @jma please add anyone else in this PR.

This is the PR to handle dates. I am still fixing tests. I an a few words:

• there is no easy way to convert dates from string to Python in the marshmallow loader and then simply convert back to string when writing to db via SQLAlchemy. We explored solutions with Invenio Architects but it is not straightforward and we might add support to Invenio in a near future.
• I decided to convert dates just before starting a loan transition, so that we can do date operations (sum/diff/timedelta/comparisons) and then convert back to string in UTC ISO format at the end of the transition just before committing to DB.
• I have updated all tests and added a marshmallow loaded to validate input dates but deserialize them as string
• We have decide to add request_start_date: the use case is very similar to the start_date of a loan, to avoid to rely on the created that can be not aligned with the real creation of the record (e.g. loans request in the past)
• affected fields: as dates only, start_date, end_date, request_start_date and request_expire_date, as datetime transaction_date
• I have removed transaction_date as required param and set it to utcnow if not provided.
• I am testing it with our ILS.