Adding "page" to gradeable component for PDFs

[wangj35]

• Closes Labeling submitted pdf with location of each component #1261.
• New "page" variable for gradeable components.
• In create/edit gradeable, the instructor specifies whether the gradeable is a PDF with pages assigned to components. If yes, the instructor also specifies who assigns pages (instructor or student).
• If instructor assigns the pages, for each component there is input for the page number.
• If student, on the homework submission page there are text boxes in the format
  question title: [number input]
  for them to submit. Values cannot be left blank or less than 1.
• On TA grading, when a component that has a page is selected the PDF will reload to the corresponding page.
• The PDF chosen is the first PDF that appears under the "submissions" folder for that version. If there are no PDFs, no opening of folders/files or reloading to pages will occur.