Implement autosave in the Wiki editor

Problem to solve

When writing long-form content in a web browser, anything can happen. An accidental click and your tab is closed. A browser could crash or your computer could run out of battery. In many parts of GitLab, content that has not been submitted is saved locally to prevent data loss. You can see this in comments left on issues and MRs. But the Wiki does not preserve unsaved changes in this case.

Proposal

Extend the Wiki editor (both in raw Markdown and Rich text mode) to persist unsaved changes to a page in local browser storage so you can recover from an unexpected or accidental end to an editing session.

Availability and Testing

Test main flow using unit tests and feature specs.

SET to Update an E2E Spec with a path that goes through the verification that the saved contents of one wiki page don't conflict with another.