Global notifications

[Mystler]

Add some simple system to enable/disable and configure global alerts for whenever/whatever we need them. (E.g. to warn about important bugs or hotfix changes.)

[aethys256]

Also we could imagine a way to record the last visit (maybe base it on Cookies or do write it in local storage) and notify people when the page they see has been updated since last visit.
Maybe as an opt-in ?