Hedgedoc: websocket upgrade queries routed to grafana

Hedgedoc has some difficulties to communicate with the server with websockets. The protocol falls back to web polling causing some weird behaviors during parallel editions (lost content, duplicate characters, ...)

websocket.js:124 WebSocket connection to 'wss://hedgedoc.softwareheritage.org/socket.io/?noteId=features&EIO=3&transport=websocket&sid=<redacted> failed: Error during WebSocket handshake: Unexpected response code: 404

The cause seems to be a wrong backend routing in varnish for the upgrade requests only:

*   << BeReq    >> 73173170  
-   Begin          bereq 73173169 pipe
-   BereqMethod    GET
-   BereqURL       /socket.io/?noteId=features&EIO=3&transport=websocket&sid=<redacted>
-   BereqProtocol  HTTP/1.1
-   BereqHeader    Host: hedgedoc.softwareheritage.org
-   BereqHeader    Connection: Upgrade
-   BereqHeader    Pragma: no-cache
-   BereqHeader    Cache-Control: no-cache
-   BereqHeader    User-Agent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/100.0.4896.75 Safari/537.36
-   BereqHeader    Origin: https://hedgedoc.softwareheritage.org
-   BereqHeader    Sec-WebSocket-Version: 13
-   BereqHeader    Accept-Encoding: gzip, deflate, br
-   BereqHeader    Accept-Language: en-US,en;q=0.9,fr;q=0.8
-   BereqHeader    Cookie: connect.sid=s%<redacted>; indent_type=space; space_units=4; keymap=sublime; loginstate=false
-   BereqHeader    Sec-WebSocket-Key: <redacted>wg==
-   BereqHeader    Sec-WebSocket-Extensions: permessage-deflate; client_max_window_bits
-   BereqHeader    X-Forwarded-For: xx.xx.xx.xx
-   BereqHeader    X-Varnish: 73173169
-   BereqUnset     Connection: Upgrade
-   BereqHeader    Connection: close
-   VCL_call       PIPE
-   BereqHeader    upgrade: websocket
-   BereqUnset     Connection: close
-   BereqHeader    connection: Upgrade
-   BereqUnset     upgrade: websocket
-   BereqHeader    upgrade: websocket
-   BereqUnset     connection: Upgrade
-   BereqHeader    connection: Upgrade
-   VCL_return     pipe
-   BackendOpen    95 grafana 192.168.xx.xx 3000 192.168.xx.xx 59022 connect <---------------- wrong backend
-   Timestamp      Bereq: 1650561229.210203 0.000000 0.000000
-   BackendClose   95 grafana close  
-   BereqAcct      0 0 0 0 0 0
-   End  

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Migrated from T4191 (view on Phabricator)