BE-Register Heat

[MarisiaS]

This PR address issue #55

Implementation

1. backend/api/models.py

Implemented the Heat model with the following fields:

• event: This is a foreignKey to MeetEvent. Set on_delete to cascate.
• athlete: This is a foreignKey to Athlete. Set on_delete to set null because a lane could be empty.
• lane_num: IntegerField
• seed_time: This is a CharField to represent an athlete's seed time in minutes:seconds.milliseconds format. Could be null.
• heat_time: This is a CharField to represent an athlete's heat time in minutes:seconds.milliseconds format. Could be null.
• num_heat: IntegerField

Added a unique constrain to event, lane_num and num_heat.

2. backend/api/migrations/0015_heat_heat_unique_event_lane_heat.py

Migration file generated with command makemigrations for Heat model.