Images fixes

[bjones1]

I spent some time reviewing the new webpack-based approach for the Runestone Components to ensure all the images are correctly included. I see two problems we need to address:

1. The clock image used for timed questions doesn't render properly. I think it would be best to assign this to a style, then add CSS to reference the image directly. See all instances of ../_static/clock.png in the timed_xxx.js files, and the source image in runestone/common/project_template/_static. I'd suggest moving this to runestone/common/css then modifying the CSS these.
2. The play overlay icon at https://github.com/RunestoneInteractive/RunestoneComponents/blob/master/runestone/common/js/bookfuncs.js#L285. This relies on web2py templates to render, so it's already a non-starter for pretext. Again, put this in CSS then just set a style, I would think.

I'm not aware of any other missing images.

[bnmnetp]

1. The solution is to include the clock.png image in the webpack bundle for timed questions and use a CSS class to show or not show that image.
2. Lines 280-287 are not used anywhere anymore. They can be removed.

[zachneill]

@bekjon777 and I are working on this (Berea College students)

[Bekjon777]

Does the clock image have to be shown to all types of timed questions?