More quick fixes

- AskQuestionActivity: Upon submitting, it will finish(). MainActivity
will then immediately start the QuestionList activity for the
appropriate category. This is because hitting the back button in
QuestionList before would lead back to the AskQuestionActivity.

- QuestionView: Changed the title TextView to display "Question by:
<username>". This is because Questions as they are now do not have
titles, and it was previously just displaying the question body twice.