Move preroll chapter to bumpers directory

Prevents oversight of where preroll bumper resides when stored
within the Foundations section despite preroll used throughout
many slide decks.
Move Office Hours and Questions link from Preroll to Goodbye bumper

Clean up opening slide with logo and chatroom only. Avoids distraction
of starting information overload. End bumper includes Training class
link and Stackoverflow link.
Update StackOverflow spelling with space
Update Stack Overflow link with tags in URL
Merge pull request #217 from github/bumper-feedback-link

Update Preroll and End Bumpers