Make websocket diconnection alert more flexible and automatically clear upon reconnection

[hhaensel]

Currently, the ws connection alert is handled by adding an anchored div to the end of the document and setting the site's location to the anchor. This spills the url, moves the focus of the page and it doesn't disappear upon reconnecting.

(This issue corresponds to a Genie issue GenieFramework/Genie.jl#722)

I propose the following changes

• make the warning an overlay
• prepend an equally spaced div to the page to avoid blocking functionality of the UI, which might also work without ws
• let the user chose whether to display the above div
• let the user chose whether to display the alert
• clear the overlay upon reconnection, don't display the alert, if the reconnection was done within the webchannels_server_gone_alert_timeout
• create a autofield ws_disonnected that is updated upon ws disconnection/reconnection