Clean up schema for table events

From https://gitlab.com/gitlab-com/infrastructure/issues/1709 (private to GitLab employees):

• Missing FK: project_id -> projects (11256 entries violate this rule)
• Missing constraint: author_id NOT NULL: https://gitlab.com/gitlab-org/gitlab-ce/issues/35586

The following will be taken care of by https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/12463:

• Get rid of events.data once push events data has been moved to a separate table, since it does not appear to be used for other event types
• Missing FK: author_id -> users (8763 entries violate this rule)
• Missing constraint: action NOT NULL
• Remove the column events.title as it appears to always be NULL
  • Confirmed that as of July 25th, 2017 this column is always NULL

Instead of using polymorphic types we should use separate columns for the different target types (diff_note_id, discussion_note_id, etc). This allows us to re-use existing functionality instead of having to write our own foreign key related code. The number of columns added is not that great after all.