[Feature] Welcome Message webchat

[ptrckbp]

In settings page, add a field for an optional welcome message for users of webchat, that would show when you restart the emulator.

I've used livechat in the past and this was a feature, so I expected it to be there, and kept looking for it. This can be done via front-end code, but it doesn't feel convenient.

For reference, this is how Tawk.to does it