[Dashboard] Use SessionStorage to Persist ViewMode State

[ThomThomson]

App State Removed from URL

In 7.x, #86939 & #97941 removed all dashboard app state from the URL. This allows for a cleaner developer experience, shorter URLs for the user, and the ability to use by value panels in dashboards, but unfortunately results in some changes to the user experience.

Session Storage

A potential pitfall with removing app state from the URL was that the dashboard editing experience could become frustrating because reloading the page or leaving the dashboard app could potentially delete unsaved changes that the user has made. To mitigate this, we created a system which stores unsaved edits in the browser session storage, with the dashboardId as a key.

What about View Mode?

The session storage system only works when the user enters edit mode. This means that any filters or query changes that the user makes in View Mode will not persist over reloads, or over navigations. The session storage system could be extended to store these changes as well for a more consistent experience.

[elasticmachine]

Pinging @elastic/kibana-presentation (Team:Presentation)