Content/Spark type validations

Added validations to ensure that only Sparks with content_type L, V, C, T, P, or A are accepted into the database. I'm fairly sure that's every one we decided upon, but obviously it would be very easy to expand or change this list.

I also did the same thing with the spark_type attribute, limiting it to W, for What-If Question, P for Problem, and I for Inspiration.

BC, I also made a slight alteration to the database for these attributes, limiting each one to only one character in the database, so make sure to migrate the database when you push this to heorku.