More functional Borrowing + Validation by lender

[da-kami]

At the moment there is some logic to check if a loan is acceptable for the lender, but it is not complete. E.g. min/max principal is not checked, and we don't keep state for what interest rate packages are acceptable to compare against them.

Both Bobtimus and the frontend should at least include:

• Overcollteralization (has to be at least max_ltv but user should be able to define how much and Bobtimus logic should just ensure at least max_ltv)
• Handle term slots (up to 3)

Note: For now same interest rate for all slots is fine.