Display Click to Load YouTube settings if feature is enabled

[kzar]

The Click to Load YouTube feature has a settings in the options page,
so that the user can configure video previews. Let's start displaying
that, but only if the feature is enabled.

Reviewer: @franfaccin

Steps to test this PR:

1. Open the options page and check the YouTube preview setting isn't visible.
2. Enable YouTube Click to Load (change "options.html" in options page URL to "devtools-panel.html", select the tiny config icon at top left, select "config" in drop down, search for "clickToLoad" and set "Youtube" state to "enabled".
3. Open options page again, and check the setting is visible.
4. Test toggling the setting to ensure previews are disabled/enabled (you can use the test page here).

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

[franfaccin]

I tested the build and toggled the CTL settings and the Options setting appeared and disappeared as expected. Great job making it conditional to the feature flag ✅