Update list editor with tabs and improved UI

[tagawa]

Reviewer:
@sammacbeth

CC:
@GuiltyDolphin

Description:

The current list editor is functional, but to make it easier to use should have a better UI.
This PR uses the existing extension styles (in base.css) and replaces the list chooser dropdown with tabs, to save clicks.
See internal task: https://app.asana.com/0/0/1206280121954707/f

Steps to test this PR:

1. Once built, open list-editor.html
2. Check that the tabs change the textarea content.
3. Check that the buttons still work, e.g. remote file is loaded.

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

[sammacbeth]

Nice, it looks much nicer now.

[tagawa]

@sammacbeth Great, thank you for both of these reviews/merges.