List bots as conversation participants, allow @ mentions and interactions

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Steps to reproduce

1. Install bot by occ command : ./occ talk:bot:install "testweb" "<shared-secret>" "<webhook-url>" --feature webhook --feature response --feature reaction --feature event
2. Enable the bot in conversation.
3. Try to @ the bot in conversation.

Expected behaviour

Being able to find and interact with bots in conversations

Actual behaviour

The bot cannot be found in the chat, nor can it be @mentioned.

Insert your browser log here, this could for example include:
a) The javascript console log
b) The network log
c) ...

Start with the Talk app's bot flow and the occ talk:bot:install entry point described in the issue. Trace how installed bots are exposed as conversation participants and how @ mentions are handled; done means users can find, mention, and interact with the bot in a conversation.