Add slash command for moving an issue

What does this MR do?

Continuation of https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/8857.

Adds a Slack / Mattermost slash command to move an issue.

Are there points in the code the reviewer needs to double check?

@smcgivern todo:

• Fix spec failures
• Add specs for error cases
• Add specs for presentation

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend

What are the relevant issue numbers?

None that I could find.