DfnDriver: create meetings
add a database table to store meeting for a course
add model class for meetings
[DfnVcDriver] move code to reusable helper methods
pass all parameters to `isMeetingRunning()`
include more user information in the join params
generalise parameters

Move `identifier` and `remoteId` to the base parameter class to make
them available in all parameter classes.
[DfnVcDriver] implemented isMeetingRunning()