Schedule quality backend

[vadimkerr]

This PR adds an endpoint returning a schedule quality score, overloaded users and comments on the existing issues (e.g. balance issues or gaps).

Limitations

• Since working hours editor is not implemented yet, there are only two scores taken into account: balance score and a score representing the ratio of time when someone is on-call to the whole time period.
• Time period is now set to be constant (90 days from today), so in some cases the results will be inaccurate (when rotations don't align with the time period)
• It only takes primary rotations into account (overrides are ignored)

Usage

GET /api/internal/v1/schedules/<pk>/quality?date=<TOMORROW_DATE>

Note that date should be tomorrow date, because we can only be sure about changing tomorrow's shifts (some of the shifts for current day could be "deleted" but still show up in the UI).

Example response

{
  "total_score": 90,
  "comments": ["Schedule has no gaps", "Schedule is well-balanced, but still can be improved"],
  "overloaded_users": ["USSZ5WRH2CUA9", "U74XJZSSQGBIH"]
}

Issue: #118

[matiasb]

Changes look good. Added a few comments/suggestions, feel free to ignore them if they don't make sense :-)

[joeyorlando]

@vadimkerr should this PR be merged? (just doing a check on PRs that have been open for a few months 😄 )

[vadimkerr]

This PR needs some front end changes as well, @teodosii could you please take a look at this?

[teodosii]

It's on my list, I need some more time to finish current work and then I can get on this one.

[vadimkerr]

Some additional backend chages are needed to support this design, so converting back to draft PR.

[vadimkerr]

Merging the backend part for now, @teodosii let's have a separate PR for the frontend.