prevent duplicate talks from being created (intermediate)

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currently, the API will accept more than one talk title. update the tests and write code to only permit unique talk titles. provide a clear error message in the response.

[444c43]

@ekelkar, apologies for the long delay. i'm getting this project back and going. are you still up for working on this (your current branch is HERE)? if not, maybe @murjax would like to finish it? we should be posting regular pair programming events for RubyJax+ soon, so that would be a great opportunity to dive back in.