Add info about start and end of semesters

[dantetemplar]

Describe the feature

Currently https://sport.innopolis.university/api/semester produces:

[
  {
    "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

[dantetemplar]

Also propagate in our API so website will show "you have N days to farm sport hours"