live-branches: Allow for not installing Jetpack

[anomiex]

Changes proposed in this Pull Request:

Jurassic Ninja installs the release version of the Jetpack plugin by
default, unless a nojetpack option is supplied. Let's add a UI for
that to simplify testing of the standalone plugins without Jetpack
itself installed.

To keep the UI simple, we add "Jetpack" in the existing "Plugins" list,
pre-checked to preserve the current behavior as the default. In the JS
we invert the sense of this checkbox so nojetpack is added to the URL
when the checkbox is unchecked.

Other information:

• Have you written new tests for your changes, if applicable?
• Have you checked the E2E test CI results, and verified that your changes do not break them?

Jetpack product discussion

None. But see pdV5qK-2I-p2#comment-81

Does this pull request change what data or activity we track or use?

No

Testing instructions:

• Install the new version of the script from https://github.com/Automattic/jetpack/raw/add/live-branches-allow-no-jetpack/tools/jetpack-live-branches/jetpack-live-branches.user.js
• Load a PR. See the new "Jetpack" option under "Plugins". Verify that unchecking and checking it updates the URL as expected. Also verify that other existing checkboxes continue to update the URL as they did before.

[github-actions]

Thank you for your PR!

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[jeherve]

The 2 "Jetpack" entries and the disappearing link when you unselect the 2 checkboxes may be a bit confusing, but this works.