[Feature Request] Configurable character limit for user messages

[dlassalle]

Task

Create an additional setting in Admin panel for setting a character limit on individual chat messages.
Create a UI element in-chat that displays the remaining character limit when there is one set

Use Case

Bots and integrations that use NLP suffer from long, rambling messages from users. Limiting characters is a useful way of forcing the user to keep their inputs short and direct, increasing the accuracy of NLPs.

[dlassalle]

issue resolved by #443