Tweet this button should populate the speakers twitter handle

[DaveParr]

For speakers that supply it, we allow them to associate thier twitter account to thier bio, which is often collected during the 'Call for papers' phase when organising an event.

The 'tweet this' button should open an intent with the speakers twitter handle automatically populated into the tweet, if it is available in the relevant data/schedule/talkx.yaml file, and if not, should not fail or fill in the information.