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

Sass compilation includes the LMS's "vendored-in" CSS libraries at
lms/static/css/vendor as a source, and output CSS is written to the
parent directory, lms/static/css. This violates the constraint that
source directories cannot be written to while they are bind-mounted.

To resolve the issue, we merge the contents of lms/static/css/vendor
into common/static/css/vendor. (The directory common/static/css only
contains sources: no CSS is outputted there.)

Now, common/static/css only contains sources for the Sass build, and
lms/static/css is only used for output.

More details: openedx#32835
Part of: https://github.com/openedx/wg-developer-experience/issues/166