Fetch users by ID instead of Username

For #44 (closed) I think we need to stop fetching objects by properties other then the ID, if possible. Only then are they automatically cached. For now, the two biggest offenders are fixed, which should help things.

Jenkins Build SUCCESS

Results available at: Jenkins [Queue #174]

By Jenkins on 2017-09-13T07:33:45 (imported from GitLab project)

By Jenkins on 2017-09-13T07:33:45 (imported from GitLab project)

changed the description

merged

By Otto Visser on 2017-09-13T08:50:31 (imported from GitLab project)

mentioned in commit f3a97042

By Otto Visser on 2017-09-13T08:50:31 (imported from GitLab project)