Improve chat endpoint

[normen]

Improving the chat endpoint to work with <|SYSTEM|>, <|USER|> and <|ASSISTANT|> flags, including a model completion template for llama.

Issues: Using <|SYSTEM|> as a stopword causes llama to output an empty message (no content field), possibly due to the code in the gpt-j wrapper. Hence, a workaround has been added to remove <|SYSTEM|> messages.