Create ScheduleModel Enum

[eoyilmaz]

User Story

As the developer of Stalker I want to have the schedule model value to be a enum class so that it is more modern and Pythonic.

Acceptance Criteria

1. Create ScheduleModel enum in stalker.models.mixins:

class ScheduleModel(Enum):
    Effort = "effort"
    Duration = "duration"
    Length = "length"

2. Implement ScheduleModelDecorator to parse values coming from the db as ScheduleModel enum values.