fix brand config previewing and skipping

the actual changes are in:
ryankshaw/brandable_css@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/dist/brandable_css
* run it again, it should generate all the "stock"
  (unbranded) css and skip all the brand configs
* change a value in variables.scss
* run it again, everything should be recompiled


also fixes: CNVS-24269 and CNVS-23786

to test:
* with a canvas_cdn.yml enabled and delayed jobs running
* go to theme editor for your account
* change a value to a non-default value
* press preview
* it should generate the preview correctly
  (you should not see the giant svg icons)
* "apply" it, it should still not have the unstyled
  giant svg icons



Change-Id: I992c11b392a0d6bbb045b16a78036f5ec1afed31
Reviewed-on: https://gerrit.instructure.com/65203
Reviewed-by: Jacob Fugal <jacob@instructure.com>
Tested-by: Jenkins
QA-Review: August Thornton <august@instructure.com>
Product-Review: Ryan Shaw <ryan@instructure.com>

Author: ryankshaw

.npmrc

@@ -0,0 +1 @@
+engine-strict=true

package.json

@@ -6,7 +6,7 @@
     "node": ">=0.12.5"
   },
   "dependencies": {
-    "brandable_css": "0.0.52"
+    "brandable_css": "0.0.55"
   },
   "devDependencies": {
     "babel": "^5.8.23",