Filter muted submissions when suggesting work

Adds a unmuted_for(username) class method to submission with test and
chains that method in user to prevent muted suggestions from being mixed
in when random submissions are suggested. Fixes #747
Add CONTRIBUTING.md
Merge pull request #771 from iHiD/contribute

Add CONTRIBUTING.md with style guidelines
Merge pull request #747 from kytrinyx/filter-muted-submissions

Filter out muted submissions in random nitpick suggestion