Add upload save feature

Add /api/upload-save endpoint that streams the data via the slave
assigned to the target instance to the instance saves directory, along
with a ctl command and web interface button to do the upload.

test/package.json

@@ -10,6 +10,7 @@
     "@clusterio/master": "^2.0.0-alpha.6",
     "@clusterio/slave": "^2.0.0-alpha.6",
     "express": "^4.16.2",
+    "form-data": "^4.0.0",
     "fs-extra": "^8.1.0",
     "jsonwebtoken": "^8.2.1",
     "jszip": "^3.2.2",