Handle slow save requests better

[philippotto]

• warn the user when there are unsaved changes older than X seconds (for testing, X is set to 5 seconds; for production, I'd suggest to set it to 2 minutes)
• send a warning to airbrake when a save request takes longer than 1 minute (set to 5 s for testing)
• send a a warning to airbrake when there was a conflict state (status code 409)

URL of deployed dev instance (used for testing):

• https://debugsavetimeout.webknossos.xyz

Steps to test:

• modify a tracing and a warning at the save button should appear after 5 seconds. the back-end has a hardcoded sleep which should make the save request extra slow

Issues:

• solves front-end part of Very long-running save requests cause problems #4594

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• Updated (unreleased) changelog
• Updated (unreleased) migration guide if applicable
• Updated documentation if applicable
• Adapted wk-connect if datastore API changes
• Needs datastore update after deployment
• Ready for review

[daniel-wer]

Awesome, this is a great improvement. Worked great during testing and looks and feels very smooth :)
I added some questions and suggestions.

[daniel-wer]

Excellent 👍