- Added InternalAutoGradingTest, which is a compiled duplicate of AutoGradingTest,
  to fix issues with mixing test/main Java code and verification errors for
  submissions
- Updated build.gradle to specify Java 1.7 source compatibility
- Removed source set modifications from build.gradle that are unnecessary
  with new InternalAutoGradingTest approach
Adds application main class and task for creating submissions.

Spring boot ignores mainClassName if it is set before applying the Spring Boot plug-in. Therefore it cannot be set on the command line using the -P option. Since AutoGrading is just a simple Java executable, a JavaExec task is sufficient for running it.

This allows running the server:
gradlew run
And creating submissions:
gradlew create-submission