mutations: log the authenticated user

[oliver-sanders]

sibling: cylc/cylc-uiserver#224

Since v8.0 cylc-flow is a single-user application, the cylc-uis provides an authorisation layer enabling multi-user functionality.

When mutations are actioned from the UIS we should log the mutation and the user who actions it at the UIS but also pass the user through as metadata and record it in the cylc-flow workflow log.