Logout clears local storage including lastRead value

[craig-willis]

This only happens to me intermittently in non-private sessions, but we are explicitly clearing out local storage during logout:

https://github.com/whole-tale/dashboard/blob/master/app/services/user-auth.js#L50-L68

For the new notification implementation, this means that the lastRead value is cleared on logout, resulting in the display of all notifications after logout.

Perhaps we could either clear everything except lastRead or consider adding lastRead to the user object?

[ThomasThelen]

Probably related to #29. I like the idea of not clearing it on the logout.