This will make the mobile view show unseen state if any of the
badges have unseen state rather than not show it if one of the
badges was more recently seen than the other in desktop.
Bug: T141404
Change-Id: I27109ee6a24831d58767f9bd13ed58e54094aee1
Like count, it is available both grouped by section and ungrouped
(top-level).
Unlike count, the top-level still has both sections (but with the
string 'seenTime' at the root), and if groupbysection is used,
it will not also have top-level (since it would be redundant).
Example output at T139993
It will be false or omitted if there is no seen time, depending
on JSON format version (2+ is false).
Bug: T139993
Change-Id: I9f4f9df69203204b56002afa1be6ed2336c33898
So we're not abusing user preferences for the last seen time.
EchoSeenTime is a small wrapper around ObjectCache that handles the
fallback to user preferences during the transition.
All JavaScript code now needs to use mw.config.get('wgEchoSeenTime').
Bug: T95839
Change-Id: Ia45ba5e30eb4564250539d04d5886d2598ebd49a
