Switch to using new langchain constructs

[chriswilty]

In #897 we upgraded langchain lib to latest 0.1.* version, but without modifying our code to use the latest, recommended functional constructs. This issue is for switching over to use the LangChain Expression Language (LCEL) instead of the older, and in some cases deprecated, class constructs.

Tasks

• LLMChain can be replaced with a new functional chain construct
• RetrievalQAChain can be replaced with a new functional chain construct
• Direct use of the generic PromptTemplate can potentially be replaced with a ChatPromptTemplate combining HumanMessagePromptTemplate and SystemMessagePromptTemplate. This may be particularly important as the docs state System Chat Message should be followed more closely than a generic prompt, which should improve consistency in responses.

See the LCEL cookbook for further code examples.