Add timezone info to chat messages

It would be nice to see the sender's local timestamp of chat messages in the gui client, e.g. when hovering the timestamp of the message (which is in my local time), it would show a popup next to it with the local time & timezone of the sender.

Possible rendition:

The chat json api could convey this by adding a new sent_in_tz field, indicating just the timezone. Example:

{
    "msg": {
        "id": 52853,
        // ...
        "sent_at": 1566533288,
        "sent_at_ms": 1566533288566,
        "sent_in_tz": "GMT",
        "content": {
            // ...

The format of the value could also be like "UTC+01" or "Europe/Amsterdam" or "+0100", whatever you feel works best.

There is a slight privacy concern here; people might not want to reveal even their rough location, so a setting in the client for omitting timezone information in chat messages might be in order.

Begin with the chat JSON API and the GUI client's message timestamp hover behavior described in the issue; check how sent_at is represented and consumed. Done means sender timezone information can be conveyed and shown beside the timestamp, with a client setting that addresses the stated privacy concern.