[RCA] Allow (persistent) "re-ordering" of observations added to an investigation

[jasonrhodes]

Observations that are added to an investigation are stored in the order they were added. In the future, we may allow users to "collapse" or "stack" selected observations into a single view, but we may also want to allow users to quickly shuffle and re-order their observations so they can see charts next to each other.

Acceptance Criteria

• Observations can be persistently reordered within an investigation (requires an API endpoint to handle this update)

Notes

• This may be a two step process for a user, eventually, in the sense that they re-order some observations in the UI and don't save the changes, but are asked if they want to persist those changes to the investigation. For now, we just want to support a persistent way of doing this.
• We should think about whether this will cause problems with conflicts if multiple users are adding observations to an investigation at the same time, and one is re-ordering them while another is adding new ones, how do we handle this?

[elasticmachine]

Pinging @elastic/obs-ux-management-team (Team:obs-ux-management)