Add date_from on item loans

[CJGutz]

Is your feature request related to a problem? Please describe.
Users want to be able to plan their loan, but not necessarily start it right away. It should be possible to set when the loan will start from. This is not required so if a user does not set it, it is defaulted to the day, the loan was instantiated.

Describe the solution you'd like
The model ItemLoan tracks when the loan has started, but it is defaulted to timezone.now. This can be kept. Instead should the form be edited to include the loan_from property which is nullable. Check that the max loan time restriction still works.

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
Add any other context or screenshots about the feature request here.