Hygiene: Refactor EditorOverlay's templates

Reuse is good. Do more of it.
Simplify the editor overlays.
Note: I want to go further in a follow up patchset and remove the need for EditorOverlayBase.hogan

Change-Id: I0be68888836152fd1d2711b3a7dd8f86029ae3a4
Merge "Hygiene: Refactor EditorOverlay's templates"