Add Availability model

[rsmithlal]

Availabilities can be defined for an
available class. Polymorphic. An available class can have one or more availabilities.

Some examples of available classes include groups, people and locations.

Availabilities can be used to define opening hours for groups or locations, and can be used by people to indicate when they are available to meet.

Can be used to schedule events or meetings at locations and managed properties.

Attributes

• from * - datetime
• to * - datetime