This commit adds a blue dot next to the "What's New"
link in the admin sidebar if the user has not seen the
new features yet, as a followup to 3e5976f843
which removed the tab on the dashboard that had this same
functionality.
When the admin visits the "What's New" page they count
as having seen all the features straight away. This could
be something we want to change, but for now this keeps the
same functionality.
We used to show New Features in a tab on the dashboard,
but this could get pushed down the page especially on
our hosting. In 043117ca13
we made a separate What's New page, so this commit removes
the dashboard tab and changes the admin notification to
send the admin to /admin/whats-new instead of the dashboard
tab.
