Add Event and EventInterface classes

The Event class provides the base class for objects aiminf at being
displayed as a calendar event in the calendar views provided by
adesign/calendar-bundle.
Add the LoanEvent class

LoanEvent is a subclass of Event and provides an easy way to bridge
Loan entities and calendar Events, as required by
adesign/calendar-bundle. The class defines a convenient factory method.
Update the Event Listener to use the LoanEvent object

The Event Listener does not have to set the attributes of generic event
objects anymore. It can now rely on the Loan Events methods to build
events straight from a Loan entity.
Refactor calendar-settings.js to use constants

Move string constants to the header of the file and use them
consistently throughout the code.
Update LoanEvent to wrap a Loan entity

LoanEvent objects now use a Loan object they hold as an attribute to
compute the other attribute values at runtime, such as the CSS classes.
Add CSS Styles for calendar events

Add the styling for the pending, accepted and refused event views on
the calendar.