Matrix question width

[durraniu]

This is to address #233. Following changes are made:

• matrix_question_width and matrix_option_width are added to sd_question to modify the width of the question and option columns in the type = matrix questions.
• Since a matrix table can be quite tall with lots of questions, I thought to make the header row sticky so that respondents keep seeing the options while scrolling down. I gave 70vh height to .matrix-question-container that creates a scrollbar for the matrix table. Not sure if this is the best option but it does the job.

[jhelvy]

Just noting these are on my radar, but I'm slammed with grading and such at the end of the semester. Will look through it in the next week or so.