Clean up schema for table audit_events

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

  • Wrong data type: created_at should be timestamptz
  • Wrong data type: updated_at should be timestamptz
  • Missing FK: author_id -> users (?) (351798 entries violate this rule)
  • Missing constraint: created_at NOT NULL
  • Missing constraint: updated_at NOT NULL
  • Missing constraint: details NOT NULL
  • "Polymorphic" table prevents regular FKs, use triggers or change to independent columns or non-polymorphic tables
  • Missing trigger for FK check: entity_type = 'Group', entity_id -> namespaces.id (4575 entries violate this rule)
  • Missing trigger for FK check: entity_type = 'User', entity_id -> users.id (346884 entries violate this rule)
  • Missing trigger for FK check: entity_type = 'Project', entity_id -> projects.id (32192 entries violate this rule)
  • Missing constraint: CHECK (entity_type = 'Project' OR entity_type = 'User' OR entity_type = 'Group')
  • Missing constraint: entity_type NOT NULL
    • 'type' is only ever 'SecurityEvent' 
      •   Removing the 'type' column would reduce the size of the table by 168MB
      •   Replacing 'type' with an enum or 4-byte integer would reduce the size by 126MB
    • 'details' appears to actually be structured data 
      • Average length of 92 bytes
      • Storing as proper columns would reduce the size of the table

We should probably move the polymorphing columns to separate columns, given I don't see us adding more than 3. These columns (group_id, user_id, and project_id) should have a foreign key and a constraint that ensures only one can be set for every row.

Further, we should also nuke the SecurityEvent class and just use AuditEvent, removing the need for the type column (saving space in the process). SecurityEvent is currently empty so there's no need for it to exist.