Check for and flag outdated images

[bwateratmsft]

Fixes #1493. Looks like this:

[bwateratmsft]

@BigMorty:

Couple questions for you that @karolz-ms and I were discussing.

1. There's two ways to decide if an image is out-of-date:
   1. If it's anything other than the latest image for this tag (i.e. compare their IDs), it's out-of-date.
   2. If it's more than X amount of time older than the latest image for this tag, it's out-of-date. If we want to do this instead, what is X?
2. Refreshing--should a manual refresh trigger a new outdated check or is once per session good enough? Alternatively, should we offer a command to trigger a new outdated check?
   1. NOTE: if you pull a new version of an image, it will automatically no longer show as "out of date"--so an outdated check is not needed just to make the yellow icon go away on an image that is now up-to-date.
3. On or off by default?

[BigMorty]

1. ImageID
2. Start with once per session and see the feedback
3. On by default, can always turn this off if we get feedback

[karolz-ms]

@bwateratmsft is this ready for review?

[bwateratmsft]

@bwateratmsft is this ready for review?

@karolz-ms Yes, I think so