Investigate the feasibility of enabling per-user dark mode

[azasypkin]

Summary

Currently, the "dark mode" setting applies for all users per space and only users with access to advanced settings can set it. In the scope of this issue we'd like to investigate the feasibility of making this specific setting applicable on the individual user level, to power a more personalized UX.

Eventually we want to have a number of user-level settings (see more details and mockups here), but having just one "dark mode" setting before that would be already immensely useful to our users. Ideally we should end up with a proof-of-concept that would help us to understand technical complexity of such change and how this change would align with our long-term vision on user-level settings.

[elasticmachine]

Pinging @elastic/kibana-security (Team:Security)

[ryankeairns]

For reference, Cloud currently displays this setting in the user profile menu. Whether we use this or some other design, we should insure both are aligned.

[legrego]

@kc13greiner do you feel that the "feasibility investigation" has completed? If so, would it be worth closing this with a summarization of your findings, and open followup issues with the discrete work items that we need to move this forward?

If we're still working on the investigation, would you mind editing this issue description to outline the completed & remaining tasks?

[kc13greiner]

Summary of findings:

1. Per user Dark Mode can be accomplished by adding a new UI Settings client to core's Rendering Service
2. This new client will read from a UserSettings Service set from the Security Plugin
3. The value for this specific setting will be stored in the User Profile, future user specific settings may be stored in SO
4. Migration plans have been suggested so that in the future, if settings need to be centralized in SO, there is a plan in place.
5. The current Cloud implementation of 'Night Mode' will not work for our purposes, it stores the value in the Cookie, we would like the value to be stored in ES.