Switched over to a Installable App

No longer a legacy packaged app, now it is a Installable (Packaged) App
to enable the Chrome.Identity API. Downside is we can’t use content
scripts anymore, so I moved the Sheets UI menu to Citable 2.1.2

Identity is working but calling background is not.
Working background communication

Used chrome.runtime.getBackgroundPage() instead of
chrome.extension.getBackgroundPage() to get the reference to the
background page.

TODO: Can we launch Printable from Citable? Once we have that
connection working we can start rebuilding Printable.
Working external message passing

Switched from chrome.extension.sendRequest to
chrome.runtime.sendMessage &
chrome.runtime.onMessageExternal.addListener() to do cross extension
messaging between Citable and Printable.
Launches Printable onMessage

Working message passing to launch Printable from Citable.

For some reason it throws an error related to not being able to define
the background page when it is triggered from the Sheets UI. Odd. The
background page definitely gets the message and it seems to launch.

Need to pass the docId in the message.