Leetcode Difficulty Rating par boenshao

Replace LeetCode problem's difficulty with a more precise contest rating from zerotrac.

Support both "leetcode.com" and "leetcode.cn".

The green/yellow/red text color is preserved, so you can still tell the official difficulty.

*** Problems in 1st-62nd weekly contests and problems that did not come from contests don’t have zerotrac rating data and "N/A" is shown by default. ***

To show the original rating, click the extension icon on the top right of the browser and disable "Show N/A if no rating is available".

Optionally, ratings missing from zerotrac can be looked up from clist.by. In the extension popup, turn on "Look up missing ratings on clist.by", paste your clist.by API key, and allow access to clist.by when the browser asks. Ratings from clist.by are shown with a c suffix (e.g. 1275c).

Both sources use Elo: a problem's rating is the rating of a person who solves it half the time. clist.by counts people with more past contests more heavily, and also looks at how well each person did in that contest. So a c rating and a plain rating can differ for problems of the same difficulty.

For more details and source code, please visit:
- https://github.com/boenshao/leetcode-difficulty-rating-extension