Servantify Galley's DELETE /i/user endpoint

[mdimjasevic]

This Servantifies the DELETE /i/user endpoint in Galley.

Checklist

• The PR Title explains the impact of the change.
• The PR description provides context as to why the change should occur and what the code contributes to that effect. This could also be a link to a JIRA ticket or a Github issue, if there is one.
• changelog.d contains the following bits of information:
  • A file with the changelog entry in one or more suitable sub-sections. The sub-sections are marked by directories inside changelog.d.

[pcapriotti]

Looks nice and straightforward. Minor nitpick below.