Added static file limit handling in Files API

refs https://github.com/TryGhost/Toolbox/issues/114

- Just like with media files same limits apply and can be configured through the usual limit service's config

core/server/api/canary/utils/validators/input/files.js

@@ -0,0 +1,7 @@
+const limitService = require('../../../../../services/limits');
+
+module.exports = {
+    async upload(apiConfig, frame) {
+        await limitService.errorIfIsOverLimit('uploads', {currentCount: frame.file.size});
+    }
+};

core/server/api/canary/utils/validators/input/index.js

@@ -31,6 +31,10 @@ module.exports = {
         return require('./media');
     },
 
+    get files() {
+        return require('./files');
+    },
+
     get settings() {
         return require('./settings');
     },