Clear save timeout handler when unmounting the editor. Fix #208

[thibaudcolas]

Fixes #208. This is very easily spotted in Storybook by rapidly moving between different stories.

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• [x] Stay on point and keep it small so it can be easily reviewed. For example, try to apply any general refactoring separately outside of the PR.
• [x] Consider adding unit tests, especially for bug fixes. If you don't, tell us why.
• [x] All new and existing tests pass, with 100% test coverage (npm run test:coverage)
• [x] Linting passes (npm run lint)
• ~[ ] Consider updating documentation. If you don't, tell us why.~ Not relevant
• [x] List the environments / platforms in which you tested your changes. Chrome and Safari on macOS

Thanks for contributing to Draftail!

[github-actions[bot]]

	Warnings
:warning:	This pull request updates the library. Should the documentation be updated as well?

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