build: run ShellCheck Adds a ShellCheck check to edx-platform PRs and master, using the shared workflow & template from the .github repo. This will become a "required" check once it passes for 2 straight weeks on master. Brings all existing shell scripts into compliance with ShellCheck.
common ------ This directory contains common code shared between LMS and CMS, such as Mako templates, CSS, and Coffescript.