fix(deploy): move database rules to separate file

- resolves #51

Author: r-park

firebase.json

@@ -1,13 +1,6 @@
 {
   "database": {
-    "rules": {
-      "tasks": {
-        "$uid": {
-          ".read": "auth !== null && auth.uid === $uid",
-          ".write": "auth !== null && auth.uid === $uid"
-        }
-      }
-    }
+    "rules": "firebase.rules.json"
   },
 
   "hosting": {

firebase.rules.json

@@ -0,0 +1,10 @@
+{
+  "rules": {
+    "tasks": {
+      "$uid": {
+        ".read": "auth !== null && auth.uid === $uid",
+        ".write": "auth !== null && auth.uid === $uid"
+      }
+    }
+  }
+}