Introduce EditorOverlayBase 

Add common base overlay component for VisualEditor
and wikitext editor.

Added handling for captcha support on saveError

Teased apart duplicate functionality into EditoOverlayBase class
EditorNew and VisualEditorOverlay should share a base template.

Dependency: I036e4f2129d929db0a3b9a4baa87c946a4b194a9

Change-Id: I3a09351b4f7c96e39b48a4ffdb4309b9bf494d2d