[Bug 1920836] Re: Show Extended Security Maintenence status

[Robert Ancell]

This change is based on the expectation that UA tools will be the
version that is about to be SRUd. The old version gracefully fails with
`ua status --format=json`, so should already effectively return (False,
False). I didn't do an explicit check on the version as it's non-trivial
to do and I'd be more worried about introducing a bug in the version
number parsing.

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https://bugs.launchpad.net/bugs/1920836

Title:
  Show Extended Security Maintenence status

Status in software-properties package in Ubuntu:
  New
Status in software-properties source package in Xenial:
  New
Status in software-properties source package in Bionic:
  New
Status in software-properties source package in Focal:
  New
Status in software-properties source package in Hirsute:
  New

Bug description:
  [Impact]
  There is not currently a graphical method of determining if a system is subscribed to [Extended Security Maintenance](https://ubuntu.com/security/esm) updates. This is resolved by adding some [new UI](https://wiki.ubuntu.com/SoftwareUpdates#Extended_Security_Maintenance) to the software properties application.

  [Test Case]
  1. Install latest version of Ubuntu advantage:
  $ sudo add-apt-repository ppa:ua-client/stable
  $ sudo apt update
  $ sudo apt upgrade
  2. Open Software Properties
  3. Go to Updates tab.

  Expected result:
  Information is shown that indicates if this system is using Extended Security Maintenance updates, when updates will supported until, and a link to upgrade to ESM.

  Observed result:
  No ESM information currently shown.

  [Where problems could occur]
  - Software properties could hit a bug getting a response from the ua app. The current code carefully checks if and what is returned, falling back to a safe default behavior.
  - Launching software properties could trigger a bug in the ua app.
  - Software properties could show incorrect information, causing confusion for the user. The solution uses information from distro-info and the ua app which means software-properties contains no data about ESM, and instead relies on these apps that can be updated if things change.

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