Improve the getTab, getCurrentTab, etc logic.

[kzar]

The getTab, getCurrentTab, etc logic was quite confused. Let's tidy that up as best we can for now:

• Remove the unused getTab and getCurrentTab message handlers.
• Remove the unused and outdated utils.getCurrentURL function.
• Remove the unused callback parameter from utils.getCurrentTab and add a JSDoc comment for the function.
• Move the logic for getting a Tab Object for the currently focused tab into the TabManager class.
• Note the current tab ID in session storage (in memory), so that the current tab can (usually) be determined even when the developer tools or popup UI are open.

Reviewer: @sammacbeth

Description:

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