[Merge] ~lamoura/update-notifier:update-esm-handling into update-notifier:master

[Chad Smith]

I think we still want ESM-related content (package counts, what services to get etc) surfaced on Xenial/Bionic/Focal even before entering ESM right so that customers can setup those services before the "support" expires.

Here's a diff containing  what commments I mentioned inline.

https://pastebin.canonical.com/p/PK5tqymw7D/

I'll keep poking around as I'm testing.
What do you think?
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https://code.launchpad.net/~lamoura/update-notifier/+git/update-notifier/+merge/401002
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