Add "Typing" message for automatic replies

[svenseeberg]

As long as we expect automatic replies, we should show a placeholder message with 3 dots (...). This is the default indicator that the other side is typing. This message should be shown for about 60s when the last message is a user message. After 60s the message should disappear.

This is not a perfect mechanism but should do the trick in most cases. It will provide a hint to the user that an answer can most likely be expected. We have no good indication (yet?) for the front end that the back end is still working on an answer.

In the long run we probably want to indicate that the back end is actually working on an answer. However, this is only in reach when we merge the chat back end into the Integreat CMS and start implementing websockets.

Additional information

Tracking issue in integreat-chat: digitalfabrik/integreat-chat#120