Restore PUT /connections endpoint

[pcapriotti]

This endpoint shouldn't have been deleted, because it's not actually possible to modify a connection using the POST endpoint.

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.
• If HTTP endpoint paths have been added or renamed, or feature configs have changed, the endpoint / config-flag checklist (see Wire-employee only backend wiki page) has been followed.
• changelog.d contains the following bits of information (details):
  • A file with the changelog entry in one or more suitable sub-sections. The sub-sections are marked by directories inside changelog.d.