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Measure how likely users open flyout if they have unseen notifications

Description

Measure how likely users open flyout if they have unseen notifications

If users are likely to open the flyout whenever they have unseen
notifications, we should preload some more resources to make those
intial openings faster instead of lazy-loading everything.

On the server-side, we will increment the MediaWiki.echo.unseen metric
whenever we serve a page when the user has unseen notifications. Then on
the client we will increment MediaWiki.echo.unseen.click if they opened
the flyout while having unseen notifications.
By comparing the two graphs, we can determine how likely users are to
click on the flyout whenever they have unread notifications, and how
useful preloading extra resources will be.

Change-Id: I14e9aa7f03d6ef275042b8a2c4cb0e5b5a64c0d7