[Rails 5] Get rid of nil values in enums

This is a follow-up from https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/19288

The problem is that couple of models currently use enum where a key is mapped to nil value. nil value is not supported in Rails 5. Because changing mapping from nil to an integer may be tricky (and will probably require a DB migration), as a short-term solution, !19288 (merged) adds a new helper method enum_with_nil which enables nil value for enums.

But it would be much better to align out enum definitions with default rails behavior and we should rather migrate nil values to an integer.