activate all lang-channels with first selection

followup to https://git.fairkom.net/chat/fairblue/-/issues/128

current

• https://git.fairkom.net/chat/fairblue/-/tags/multiple_translation_channels
• lang-channels that were activated once are kept active in the background, so revisiting the channel does not impose a delay
• interpreters have to activate every language channel, that they want to use before the session starts

goal

• on first activation of a language-channel (to speak), all other channels should be connected and kept in the background
• interpreters have to activate one channel before the session starts; they are then ready to use all channels without delay