policies: integrate mandatory constraints and policies hooks

[ntarocco]

• added mandatory constraints to transitions
• integrated pytest-invenio package

[ntarocco]

A few comments @jma and @zzacharo, since I will be on holidays and you are maintenairs:

• I have pushed in this repo to be 100% sure that you can modify it.
• I have refactored the policies, please have a look if it makes sense
• About dates: IMHO, we should parse the string date coming from REST only when we have to validate the transition based on dates with timezone support, store the date as UTC datetime object in the loan to simplify datetime operations/comparison/validation, and in the end convert to it to isoformat() when storing them in the db. This is what I have done here. I had to add a package because there is no easy way to parse a datetime from string with timezone, see StackOverflow for more details.
• I have integrated pytest-invenio
• I have created an helper for when we want to mock app configuration: unfortunately, if u don't put back the original value of a config at the end of test, this side effect could result in other tests failing.

Feel free to push/edit/modify everything here!

[zzacharo]

@jma can you review and I will fix the tests after.