GitLab

the actual changes are in:
https://github.com/ryankshaw/brandable_css/compare/8cff7...master
make sure to review that too

first thing it fixes: 
better logic for knowing what to skip in brandable_css
closes: CNVS-24039

* gets rid of any manifest files on s3.
  The only manifest file that is used is the one
  on the local disk which only includes metadata
  about the "stock" (unbranded) css files which
  are generated as part of the tarball on the
  deployer's vm.
* which means we no longer use the env var:
  UNIQUE_KEY_FOR_BRANDABLE_CSS_MANIFEST

* because the filenames on s3 include:
  <css bundle name>
  <combined checksum of stock version of this file>,
  <md5 of this brand config>
  if that css file exists, we know we don't need
  to generate or upload that file again. and we
  don't need a shared manifest on s3 to figure that
  all out.

test plan:
* with a canvas_cdn.yml file configured
* run brandable_css
* run it again, it should be fast
* rm -rf public/d...