Server: use llama_chat_apply_template (#5593)

* server: use llama_chat_apply_template

* server: remove trailing space

* server: fix format_chat

* server: fix help message

Co-authored-by: Georgi Gerganov <ggerganov@gmail.com>

* server: fix formatted_chat

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Co-authored-by: Georgi Gerganov <ggerganov@gmail.com>