Added new language column to Assessment

Part One of #52

I've never used PHP before this point so I'm unsure whether the PR does exactly what I want, but I've verified by looking inside the Databse that the column does appear in the database with a default language set to 'Java'. Currently, this shouldn't affect any other areas of the project.

Goals

• Generalize database schema for multiple types of Assessment.

Next Steps

• Update web UI to allow for defining the type of Assessment
• Create Docker image for the worker of that language (using language-specific build tool)
• Create XXXBuildJob Laravel job class that runs from inside the Docker image and collects results
• Tell Assessment to pick the right one for submissions / model solutions