MDL-41031 events: Add a base class for page view
MDL-41093 qtype match: unique index upgrade problem.

Eliminate duplicate rows before we try to add a unique index. We
arbitrarily keep the duplicate row with the lowest id.
MDL-41031 events: Add a base class for page view
MDL-40052 events: Add page view event for report_participation
MDL-40052 events: Add page view event for report_log
MDL-40052 events: Add page view event for report_stats
MDL-40052 events: Add page view event for report_outline
MDL-40052 events: Add page view event for report_loglive
MDL-40052 events: Coding style improvments