Extensions are disabled on rendered markdown view

Created by: muratsu

If you visit a sourcegraph page with markdown today, the extensions by default are inactive. The reason is behind this is that we don't consider the rendered view to be a code editor and extensions currently don't work without a code editor.

This is confusing behavior for users who aren't familiar with how extensions work behind the scenes and it shouldn't be a blocker for some extensions like open-in-x.