Initial extension manager dialog/view/model (with non-final layout)
Update comments to remove the Mustache note, since these can work with Handlebars too
Code review fixes.
Renamed ExtensionMgr to ExtensionManager
Tweaked dialog so it's centered a little better
Move HTML templates to htmlContent
Merge pull request #3443 from adobe/nj/extension-list

Initial extension manager dialog/view/model (with non-final layout)