Constrain upload file types for scribing questions.

Only allow GIF, PNG, JPEG image files to be uploaded, in addition to PDF
files.

Downcase content type before checking.

If file is of an invalid type, redirect to new scribing question page
and display an error flash notice.

Switch order of check of file_save_success so spurious questions won't
be created when file upload fails.

References Coursemology/coursemology.org#277
Allow users to pick line colour when scribing.

Add input type=color box to row of buttons above each canvas. This
doesn't work on iOS and falls back to a simple textbox :(

Add event handler to change Fabric.js brush colour when the input value
changes.

Add styling for the input box to make it look more like the other
buttons.

References Coursemology/coursemology.org#277.
Refactor button event handlers to use jQuery's method of passing event
data.

References Coursemology/coursemology.org#277.
Merge pull request #6 from fonglh/scribing-qns

Constrain upload file types for scribing questions. Add colour picker