Add info about start and end of semesters 

### Describe the feature

Currently https://sport.innopolis.university/api/semester produces: 
```json
[
  {
    "id": 6,
    "name": "F20"
  }
]
```

and we can't calculate how many days to the end of semester.

### Suggested solution

Just add start and end fields to the output of the endpoint. 

### Additional context

_No response_