Add ability to have custom dimensions for course and activity

[brendanheywood]

Rough tasks:

• on the matomo side or ga side create a new custom dimension. Usually this is a slot number like 'slot 5'
• in the plugin have a generic dimension configuration say you can say 'I want slot X to be filled with data Y'. Later there will be many types of Y but for now it will just be the context hierarchy
• manually configure a plugin eg 'slot 5 is the context'
• implement the logic for this. The context will be the $PAGE->context which is a set of numbers but transform each number into human readable words like the course code, activity idnumber etc

Double check that including the whole hierarchy in a single value is going to work. We may instead need 1 dimension for course, and 1 for activty, etc