Update speaker request text and links

This is some early work on updating the speaker request workflow for users. This cleans up some language and adds new Google Analytics event tracking code to the speaker requests buttons to reflect a new GA Goal.

* Cleans up speaker request text to not include reference to Jen's talks by default
* Centralizes speaker request text into a partial in _includes
* Creates Google Analytics onClick event when users click the Request Speaker button to track user acquisition and flow

/cc @Jaime-Alexis please check out the language in the new _includes/speaking-requests.html partial and sign off

/cc @migurski please idiot check my liquid template code and Google Analytics event, looks good on my end