Merge issuable "reopened" state into "opened"
What does this MR do?
This merges the state
opened for issues and merge requests.
Why was this MR needed?
Having two "opened" states makes it harder to query and index data, leading to less than ideal performance. More information can be found in https://gitlab.com/gitlab-org/gitlab-ce/issues/27165#note_35375430 and the comments following it.