Auto install bundle

Automatically install extensions "bundled" with installer on startup.

**Issues**:
* Should appReady in ExtensionManger be used instead?
* Terminology: folder name and location

cc @dangoor @peterflynn 

This is for #9233 
For the folder name, something a little more self-explanatory might be good... e.g. `auto-install-extensions`. ("bundle" is a term we haven't really used for extensions before, anyway).

Also might be good to move the folder name into a constant...