Compile SCSS in CI, stop tracking CSS files in git

building off of #27 (closed)

problem 1: building SCSS is hard for many newcomers, and can even be a source of frustrations for people with more experience with our workflow ("oops, i forgot to run sass when i changed the scss!").

problem 2: tracking the built CSS in git is a bad idea. every rebuild creates enormous diffs which make the git history almost unreadable, and also inflates the size of the lego repo, which affects the build and development time for every website. we could also find ourselves in a situation where the SCSS is changed and committed, but the CSS isn't.

both of these problems can be solved by making lektor build the SCSS instead of having developers do it manually. #27 (closed) was a poorly thought-out implementation and its related RFC has been rejected. this ticket is an attempt to do things the right way.

the lektor-scss package is already in lego, so here are the next steps:

1. add lektor-scss config files to the different website repos
2. have CI build the SCSS
3. stop tracking the built CSS in lego