Added multi team support

[twolfson]

This PR adds back multi team support with the necessary edits to support tray notifications which are now on master. The key differences from #27 are:

• Renamed chat-loaded to teams-loaded
• Added notification handler for count against all windows
• Moved to addEventListener for iframe load listening
• Moved notification listener logic into SlackWindow

/cc @wlaurance

[twolfson]

Oh, and as mentioned but not addressed in #27, we should really consider embracing a framework because the code is beginning to get unweildly.

[andrewelkins]

I'd love to see this pulled in. It's actually the one item holding me back from installing.

[twolfson]

Ugh, merge conflicts again? Patching this. One sec =/

[twolfson]

Alright, pushed. We should prob wait for CI to approve.

[twolfson]

Yay, it's green =)

[wlaurance]

Hey, so what exactly should I be seeing for multi-teams? Should I be seeing the side-bar on the left?

[twolfson]

Whoops wrong screenshot. Please don't share that one -_-;;

[wlaurance]

OK, I see it now.

Does this need a MAJOR or MINOR version bump?

[twolfson]

This is backwards compatible so minor works fine.

[twolfson]

To fill in what the redacted comment had:

The multi-team sidebar will show up once users have logged into multiple teams and restarted the app.

It's hard to manage state without an framework so this was the low-tech approach for now.

[wlaurance]

OK cool. v1.4.0 is published