Don't sort sites when adding or removing them

[ayumi]

• sites is an OrderedMap sorted initially when the app starts.
• addSite sets site order to sites.size or retains the original order – in both cases iteration order is preserved.
• removeSite removes a site, also preserving iteration order.

Fix #9427

Auditors: @darkdh @bsclifton

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).
• Tagged reviewers and labelled the pull request as 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

[darkdh]

lgtm. let's wait for the test before merging it

[ayumi]

no new failing tests

[darkdh]

cc @bsclifton

[bsclifton]

Noticeable improvement when visiting sites. I have maybe 10,000 bookmarks in my test account and it's working great. I went back and compared against master to verify the speed increases.

Great job 😄 👍

[bsclifton]

I think this makes sense to pull into 0.17.x. What do you think, @bbondy?