Changes their endpoints and remove the user id from the URLs.
Simplifies list changes to share a single endpoint, which aligns it to
the behaviour of the existing sort preference endpoint.
Also added test to ensure user preferences are deleted on user delete.
Done a little reorganisation while there of misplaced tests.
Moved MarkdownTest to a new PageEditorTest to avoid confusion with
other markdown elements and to align with other page tests.
Also allowed config-centralised default user settings for this change
and bought existing user-level view options into that default settings
system to be cleaner in code usage.
For #2081
