Add-digital-channel-to-missed-collections-by-channel

Add Digital channel data series to Missed Collections by Channel graph
to provide complete 'picture' of reported missed colelctions
NOTE: this channel only covers webform based requests - social media and
mobile app requests are handled manually and so classed as 'other'
Merge pull request #348 from alphagov/Add-digital-channel-to-missed-collections-by-channel

Add-digital-channel-to-missed-collections-by-channel