Add a close issue slack slash command

What does this MR do?

Adds a new slack slash command to close issues.

Format:

• /whatever-project issue close 123345456
• /whatever-project issue close #123345456

Related Issue: https://gitlab.com/gitlab-org/gitlab-ce/issues/64830

Testing

Feature setup docs:

• https://docs.gitlab.com/ee/user/project/integrations/slack_slash_commands.html
• https://docs.gitlab.com/ee/integration/slash_commands.html

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers