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

Remove the trailing slashes from the .gitignore entries for static asset
build artifacts. With these slashes, only directory contents are
ignored. Without these slashes, the artifact is ignored whether it is a
directory *or* an actual file (particularly, in our case: a symlink).

This allows us to build edx-platform static assets into a separate
directory, and then symlink to them from edx-platform.

Also: We remove the duplicate cms/static/css/ gitignore entry.

Backport of: 1f41529