Client update reminders when the running build is really out of date

Some people are running out of date builds of the desktop client, and according to Hampus there is even someone who is running the very first build of Fluxer. hampus_comment.png

A possible solution for outdated clients can be achieved with a reminder in a top banner like this mockup: proposal.png The banner would only show up if the client build running is very old, like 1 or 2 months old or if there are breaking changes compared to the server build. This would encourage people to keep their client up to date to minimise issues with mismatched versions, bugs already fixed but not yet deployed on the user machine or missing features.