convert to "Gmail Add-On"

[rosulek]

See whether this is feasible / desirable:

https://gsuite-developers.googleblog.com/2017/10/gmail-add-ons-framework-now-available.html

[rosulek]

Observations so far:

• "an add-on can use a time-driven trigger once per hour at most" source

• no calendar widget, would have to be somewhat dumb text entry

• I don't know where the metadata associated with a thread would be stored (if not in a label)