Stop propagation for all keys but exempt the save button

This fixes an issue where if using contenteditable (default on mobile)
and you typed "/", it would get intercepted within the app.

Change-Id: I3bdf83bd64595c6238bb46736104fd0e668100c0
(cherry picked from commit 6a27089c51237743a8e7cae03837f921b0028b3b)

gr-editor/codemirror-element.js

diff --git a/gr-editor/codemirror-element.js b/gr-editor/codemirror-element.js
index ff6785d..c49d6a1 100644
--- a/gr-editor/codemirror-element.js
+++ b/gr-editor/codemirror-element.js
@@ -80,6 +80,13 @@
           new CustomEvent('content-change', {detail: {value: e.getValue()}})
       );
     });
+    this._nativeMirror.getInputField().addEventListener('keydown', e => {
+      // Exempt the ctrl/command+s key from preventing events from propagating
+      // through the app. This is because we use it to save changes.
+      if (!e.metaKey && !e.ctrlKey) {
+        e.stopPropagation();
+      }
+    });
   }
 }