ci: share yarn cache on Circle

On a cache miss, `yarn install` via `make node_modules` takes about 42
seconds, plus about 19 seconds to upload the cache. See
https://circleci.com/gh/influxdata/platform/1872.

On a cache hit, `make node_modules` takes about 7 seconds to download
the cache plus 19 seconds to `yarn install`. See
https://circleci.com/gh/influxdata/platform/1877. Using the cache here
is a net gain of about 16 seconds on cache hit, which is the most likely
case given a random commit.

.circleci/config.yml

@@ -6,6 +6,21 @@ jobs:
     working_directory: /go/src/github.com/influxdata/platform
     steps:
       - checkout
+
+      # Run yarn install, using Circle's cache if applicable.
+      - restore_cache:
+          name: Restore Yarn package cache
+          keys:
+            # Only cache on exact yarn.lock match, as in Circle's yarn example:
+            # https://circleci.com/docs/2.0/yarn/
+            - chronograf-yarn-packages-{{ checksum "chronograf/ui/yarn.lock" }}
+      - run: make node_modules
+      - save_cache:
+          name: Save Yarn package cache
+          key: chronograf-yarn-packages-{{ checksum "chronograf/ui/yarn.lock" }}
+          paths:
+            - ~/.cache/yarn
+
       - run: make test-js
 
   gotest: