improve loan status: reservation | active | overdue | completed

[torbengb]

Currently, 'status' is just a BOOL in LOANS that tells that a given loan is active (not returned yet).
Need to change this:

• Tool itself needs a (redundant?) status [AVAILABLE | LOANED] to indicate whether it's free to loan.

• If the tool status is AVAILABLE then show the LOAN button.

• If the tool status is LOANED then show the RESERVE button (waiting list). Perhaps give an indication like MAX(agreedend)?

• Action LOAN sets the tool status to LOANED, then creates an entry in table LOANS.

• Action RESERVE creates an entry in table LOANS (#TODO later: the owner should be notified).

• Action RETURN ends the current loan, then checks for any reservations and picks the one with MIN(agreedstart), then sets that loan to status ACTIVE (#TODO later: the new lender should be notified). If no next loan then set tool status to AVAILABLE.