Show timed icon clock

[zachneill]

This pull request fixes issue RunestoneInteractive/rs#64

The clock image now renders properly in all types of timed questions. Here are the changes we made to accomplish this:

• created a new time-tip.css file with the timeTip class that is added in the timed*.js files
• timeTip class add the clock image as a background-image
• we added the new css file to the webpack bundle.
• fixed all time*.js files to pass correct elements to the renderTimeIcon() function, so that the image is added to the proper div
• changed style in activecode.js shifting "alert" and "alert-warning" class to the parent element of the question box, so that the question style is similar to other question types and the clock-image appears inside the box just like in other questions.

For the second part of the issue, we removed the redundant code as suggested in the solution.