Add "Loading...", visible on congested networks

[jidanni]

How to reproduce the issue?

Have your network slowed down by a big download in another window.

Now on OSM.ORG click "edit".

After a while all you see is a white page. No, the browser isn't showing the page as still loading either.

Then after a few seconds the editor starts showing up.

But during those few seconds there should be "Loading..." shown.