Add initiator_id field to EventLog

https://www.agileplannerapp.com/boards/173808/cards/3442

an initiator would represent the user who
initiated the event for a user. useful to
find out who administers a user account.
EventLog optionally records an event initiator

https://www.agileplannerapp.com/boards/173808/cards/3442
Save and show the initiator of a user event

https://www.agileplannerapp.com/boards/173808/cards/3442
Modify integration tests to check event initiator

www.agileplannerapp.com/boards/173808/cards/3442
Merge pull request #236 from alphagov/add-initiator-to-account-access-logs

Add initiator to account access logs