Add additional string match mode to Short Answer

[clpetersonucf]

Requested by an instructor: Short Answer nodes could include an even more "fuzzy" detection mode that will match substrings, if the user response contains any of the responses for each answer set. For example, if an answer set contains black and a user enters I love black cats, the corresponding answer will be selected.

Something to consider: there's no checks of this type currently, but we should also consider what happens if there are duplicate matches in different answer sets. This is especially likely to occur with substring matching. Do we check (and prevent) duplicates? Do we simply indicate that the first match is selected?

The UI could be tweaked slightly to better indicate that there are three tiers of fuzziness to short answer response matching, from least to most strict:

• Substring matching ("Response must contain an answer")
• Fuzzy string matching ("Response must match answer, but symbols and whitespace are ignored")
• Strict string matching ("Response must exactly match an answer")

[clpetersonucf]

Implemented as part of #81