feat(chat-message): allow for custom message definition

[faust64]

Fixes https://github.com/aau-zid/BigBlueButton-liveStreaming/issues/115

Allows overriding the default message prefix, letting other attendees know bbb-livestreaming joined their conference.

[mtsonline]

Hi and thanks for the improovement. this will break the current behaviour as on updates without changing the config the anouncement will no longer be made. Still I think it is useful. Could you change it a bit for backwardscompatibility and...

• add an example to the docker-compose.
• chane the way it works to: use the old default string unless a new one is set
• disable the anouncement if the variable is set to false.

and could you do the same for the anouncement of the chat user in chat.py? It should work the same way for this anouncement.

thanks a lot cheers

[faust64]

isn't tmp_chatMsg = os.environ.get('BBB_CHAT_MESSAGE', "This meeting is streamed to") supposed to keep the default message, if none was set? added docker-compose / env examples, and some way to turn announcement off. and some equivalent for chat.py