Send user email with link to newly created issue or merge request when creating by email

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Description

• At the bottom of the project issues list page, you can click a link to view a modal with a special unique email address that allows you to create an issue by emailing in content.
• Similarly, there's a link at the bottom of the project merge requests list page with similar information for creating a merge request by email.
• In both of these flows, GitLab should respond with an email back to the user.
• The response of the email should:
  • Indicate success or failure of the issue / merge request creation.
  • If success, it should have a link to the issue / merge request.
  • If failure, it should maybe have some context / reasoning of the failure.

Original description

Per @smcgivern we currently do this for Service Desk, but not yet for regular issue by email creation. We totally should though.