Replace the component bounded cachedValue by a global UnsavedEdits

This new draft saving system is currently only implemented for the
card description and comment. We need better a component
inheritance/composition model to support this for all editable fields.

Fixes #186

collections/unsavedEdits.js

@@ -0,0 +1,34 @@
+// This collection shouldn't be manipulated directly by instead throw the
+// `UnsavedEdits` API on the client.
+UnsavedEditCollection = new Mongo.Collection('unsaved-edits');
+
+UnsavedEditCollection.attachSchema(new SimpleSchema({
+  fieldName: {
+    type: String
+  },
+  docId: {
+    type: String
+  },
+  value: {
+    type: String
+  },
+  userId: {
+    type: String
+  },
+}));
+
+if (Meteor.isServer) {
+  function isAuthor(userId, doc, fieldNames = []) {
+    return userId === doc.userId && fieldNames.indexOf('userId') === -1;
+  }
+  UnsavedEditCollection.allow({
+    insert: isAuthor,
+    update: isAuthor,
+    remove: isAuthor,
+    fetch: ['userId']
+  });
+}
+
+UnsavedEditCollection.before.insert(function(userId, doc) {
+  doc.userId = userId;
+});