Avoid iterating all sites on every about page

[ayumi]

Fix #9004

With 10K bookmarks, saves 350 ms every new tab!

Auditors: @bsclifton

Test Plan:

0. Check that tests pass.
1. Bookmark a page
2. Make a bookmark folder
3. Open Bookmarks manager and confirm bookmark page and folder.

Submitter Checklist:

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Ran git rebase -i to squash commits (if needed).

Test Plan:

Reviewer Checklist:

Tests

• Adequate test coverage exists to prevent regressions
• Tests should be independent and work correctly when run individually or as a suite ref
• New files have MPL2 license header

[bbondy]

nice find 💯

[bsclifton]

++! 😄