Add a BUILD_ONLY deploy mode

Summary:
This allows running `BUILD_ONLY=true ./deploy.sh` to generate a static,
dependency-free version of the site without actually deploying to the
GitHub Pages site.

Test Plan:
Run `BUILD_ONLY=true ./deploy.sh`. Inspect the temporary directory that
it emits; make sure that the site is fully usable without an Internet
connection. Note that the deploy script does not actually deploy.

deploy.sh

@@ -5,6 +5,10 @@ set -e
 # Disable this if you've already run 'npm build'.
 : "${BUILD:=true}"
 
+# Set this to "true" if you just want to save the contents to a
+# standalone application without deploying to the site.
+: "${BUILD_ONLY:=false}"
+
 # Explode the contents of this directory,
 # and the stage the following enclosed files and folders.
 EXPLODE=dist
@@ -79,6 +83,13 @@ else
 fi
 cp -r "$EXPLODE"/* "$TMPDIR"
 
+if [[ "$BUILD_ONLY" == "true" ]]; then
+    printf 'BUILD_ONLY is set; stopping here.\n'
+    printf 'Contents available in: %s\n' "$TMPDIR"
+    exit 0
+fi
+
+
 git checkout gh-pages
 cp -r "$TMPDIR"/* .
 git add "${STAGE[@]}"