Dismissible GlobalMessage

[metenn]

GlobalMessage should be dismissible. This aims to fix that.

• It won't sync between two devices for a client, the alternative requires tracking of every dismissed message for every single account, which requires more server work. For something like this - a message you see once in a blue moon - I think that's a bit overkill.
• The script is inlined in global-message.html to avoid layout shifts (or commonly "flashes") from the page loading well before the script.
• in global-message.html, the context variable global_message is also now passed through |safe to allow for more customizable messages via HTML. This isn't necessarily an XSS attack vector, as GlobalMessage is only changeable if user.is_superuser = True, and superusers already have full access to many other system components.