Store backtrace quality information in database

storage: move lob deletion event, add test
    
    Separate events.py file introduced for adding
    further events to ease denormalization
    required for backtrace quality computation.
    
storage: add denormalized backtrace quality field
    
    On-demand computation is too demanding
    making queries for hot/longterm problems
    taking way too long.
    
    This patch introduces ReportBacktrace.quality
    field which is updated on ReportBacktrace
    creation and updates.