move panel-related code to shared/

Created by: sqs

The panel will soon be shared by the web app and browser extension. To support this, this PR moves all panel-related to to shared/.

This was just a dumb move, except:

• shared/ CSS is now SCSS
• the url.ts file is full of messy utilities; I didn't spend time cleaning it up for this move, but I did merge a few different files with similar utility funcs into one