A better availability model

[gskorokhod]

It should be possible to set employee availability using a drag-and-drop recurrence UI (see #45).

We could have 2 recurrence patterns:

• PREFERRED (A person would prefer to work at this time)
• UNAVAILABLE (A person strictly can't work at this time)

The availability is as follows:

• PREFERRED (If the PREFERRED pattern applies at the given time)
• AVAILABLE (If neither applies; this represents that a person can work on this day but would prefer to work on a PREFERRED day)
• UNAVAILABLE (If the UNAVAILABLE pattern applies)

The 2 patterns should not clash; The system should be generalizable to Locations too