build: lms/static/css/vendor/* -> common/static/css/vendor

The git-ignored target directory for LMS Sass compilation is:
    lms/static/css

Unfortunately, that directory contains git-controlled directory of
vendored-in static assets:
    lms/static/css/vendor

This is a problem for a couple reasons:

1. In Tutor, we would like to make lms/static/css a symlink to an
   external location for the sake of build efficiency. This is
   impossible to do without clobbering lms/static/css/vendor and
   dirtying the git state.

2. More generally, when optimizing (or just understanding) a build
   system, it adds complexity when git-controlled source directories are
   mixed up inside git-ignored target directories.

The solution is to simply merge these vendored-in assets to another
existing git-controlled vendor directory:
    common/static/css/vendor

LMS already reads assets from this folder, so no further changes need to
be made. common/static/css is fully git-controlled, so we avoid the
complexity described above.

Backport of: 97a9f08