WL-3326 : Hook up Lessons to Mneme (rather than Samigo)

4 main changes:

Enable the “optional” profile in tool/pom.xml
Remove the bits about Assignments 2 and Yaft in opt-applicationContext.xml
Set right version numbers and dependency type in main pom.xml
Change context.xmls to use MnemeEntity rather than SamigoEntity
I'd prefer it if we weren't dependent on another locally run institutional maven repository, we've had problems in the past when local repos became unavailable.