added problem type with 0/undefined slope; refactored out embedded JS

[stephjang]

Trello bug title

• Add problem type to slope of a line exercise

Changes

• Added problem type with 0/undefined slope
• Refactored out embedded JS in
• Added data-weight=2 to the existing exercises and data-weight=1 to the new one

Not changed

• Kept the problem options un-randomized, due to referencing by array index within the Hints

[marcia]

Great job refactoring my suboptimal code :)

I made a few whitespace changes here 35c0c45 and then I swapped out "infinity" for "undefined" here 7090569. Thanks so much, and I look forward to more things coming my way from you!