Prevent duplicate sync polling

[rtibbles]

Summary

Description of the change(s) you made

• Removes my broken vendored version of createLeaderElection and reverts to using updated version from Broadcast channel
• Handles duplicate leaders
• Removes no longer necessary change polling code

Manual verification steps performed

1. Open two tabs
2. Confirm that backend requests are made only in one tab
3. Make a change in the other tab
4. Confirm that sync only happens in one tab, not both

Does this introduce any tech-debt items?

No - removes tech debt by removing wonky vendored/modified code.
Original reason for vendoring should now be obsolete, and was motivated by unnecessary future planning for service workers.

---

Reviewer guidance

How can a reviewer test these changes?

See steps above.

Are there any risky areas that deserve extra testing?

Make sure that removing the change polling isn't an issue - but we added it before we realized that __ prefixed tables were not observed by Dexie Observable.

References

Fixes #3191

May also help to prevent #3210, #3164, #3163

---

Contributor's Checklist

PR process:

• If this is an important user-facing change, PR or related issue the CHANGELOG label been added to this PR. Note: items with this label will be added to the CHANGELOG at a later time
• If this includes an internal dependency change, a link to the diff is provided
• The docs label has been added if this introduces a change that needs to be updated in the user docs?
• If any Python requirements have changed, the updated requirements.txt files also included in this PR
• Opportunities for using Google Analytics here are noted
• Migrations are safe for a large db

Studio-specifc:

• All user-facing strings are translated properly
• The notranslate class been added to elements that shouldn't be translated by Google Chrome's automatic translation feature (e.g. icons, user-generated text)
• All UI components are LTR and RTL compliant
• Views are organized into pages, components, and layouts directories as described in the docs
• Users' storage used is recalculated properly on any changes to main tree files
• If there new ways this uses user data that needs to be factored into our Privacy Policy, it has been noted.

Testing:

• Code is clean and well-commented
• Contributor has fully tested the PR manually
• If there are any front-end changes, before/after screenshots are included
• Critical user journeys are covered by Gherkin stories
• Any new interactions have been added to the QA Sheet
• Critical and brittle code paths are covered by unit tests

---

Reviewer's Checklist

This section is for reviewers to fill out.

• Automated test coverage is satisfactory
• PR is fully functional
• PR has been tested for accessibility regressions
• External dependency files were updated if necessary (yarn and pip)
• Documentation is updated
• Contributor is in AUTHORS.md

[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
No Duplication information

[marcellamaki]

looks good to me