Create ScheduleConstraint Enum

[eoyilmaz]

User Story

As the devleoper of Stalker I want to represent the schedule constraints with a proper IntEnum instance so that Stalker is more modern.

Acceptance Criteria

1. Create a new IntEnum with:

   from enum import IntEnum
   class ScheduleConstraint(IntEnum):
       NONE = 0
       Start = 1
       End = 2
       Both = 3

2. Create a ScheduleConstraintDecorator to parse values coming from db as ScheduleConstraint enum values.