Rating system update

[meooow25]

There has been a change in the rating system of Codeforces (blog here).

Summary:

• The default rating for new accounts is now 1400 instead of 1500.
• The real rating is now hidden. Instead, a fake rating is shown which is realRating - f(numRatedContestParticipations) where the values of the function f at 0, 1, ... are [1400, 900, 550, 300, 150, 50, 0, 0, ...].

This PR updates the default value to 1400, but cannot deal with the second change.

The problem:

• The real rating is not available through the API. Mike said he will make this available but it has been 2 weeks 5 months since and there has been no update to the API. The only other way to get real ratings of users for the purpose of rating calculation is to calculate the complete rating history of every user to ever participate in any Codeforces contest, which is not a suitable task for an extension.
  If the real rating is available, numRatedContestParticipations can be calculated from the difference between real and fake ratings.
• Alternately, if numRatedContestParticipations could be conveniently available for all users through any API endpoint, the real rating can be obtained from the fake rating. Unfortunately, it is not. The alternate is to either query each individual user's rating history for every participant or query the rating changes for every contest on Codeforces ever, both of which again are not suitable tasks for an extension.

TL;DR: The Codeforces API is lacking and this recent change makes it impossible to predict rating changes accurately.