File editor for web repl:
1. Create a CodeMirror-based file editor (in webedit.cljs)
2. Create webrepl3.cljs and repl3.html, which use the file editor
3. build2.sh builds the console-only version of clojurescript.net;
   build3.sh builds the version including both console and file editor.

The file editor's contents can be evaluated in the REPL by pressing
Ctrl/Cmd+E, and the editor itself can be shown and hidden by pressing
the Show/Hide file editor link.