Refactor tab to serialize to storage

[jonathanKingston]

Reviewer: @shakyShane

Description:

1. Create a new class to manage the state of tabs and sites.
   These need writing to session storage to allow us to restore their state after the service worker is reinitialized.

2. Uses getters and setters in to store into the new class from tab and site.

3. Changing the tab and site shape to have getters and setters will break the consumers of the current tab serialization also (we serialize over message passing to different contexts which turns classes into plain objects).
   So additionally we will need to manually serialize into this plain object format for sending over to different contexts; this will allow autofill, content-scope-scripts and the dashboard to retain their current usage of tab and site data.

https://app.asana.com/0/0/1202914947727988/f

Steps to test this PR:

Automated tests:

• Unit tests
• Integration tests

Reviewer Checklist:

• Ensure the PR solves the problem
• Review every line of code
• Ensure the PR does no harm by testing the changes thoroughly
• Get help if you're uncomfortable with any of the above!
• Determine if there are any quick wins that improve the implementation

PR Author Checklist:

• Get advice or leverage existing code
• Agree on technical approach with reviewer (if the changes are nuanced)
• Ensure that there is a testing strategy (and documented non-automated tests)
• Ensure there is a documented monitoring strategy (if necessary)
• Consider systems implications

[shakyShane]

Just a few small comments :)

[shakyShane]

🚀