[Fix] Small fix in mod_settings. Occuring when there are keys in diff object that are no longer being used in defaultSettings.

Surfaced because we recently stopped using 'nextCU' settings key and it was present in the localStorage diff settings.
[Refactor] Minor code improvement.
[Data] Improve data for FB, Youtube and TOI.

- FB: Improve the main element selectors. Fixes #174 and #175.
- Youtube: Fix main element selector. Fixes #177.
- TOI: Improve zen mode data.