Servantify conversation self member endpoints

[pcapriotti]

This converts /conversation/:cnv/self endpoints (GET and PUT) to servant, adds local-only stubs for their qualified counterparts, and deprecates the unqualified endpoints. This is only an internal change.

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 end-points have been added or changed: the endpoint / config-flag checklist (see Wire-employee only backend wiki page) has been followed.
• Section Unreleased of CHANGELOG-draft.md contains the following bits of information:
  • A line with the title and number of the PR in one or more suitable sub-sections.
  • If public end-points have been changed or added: does nginz need upgrade?