Rework the Preferences to prevent FOUC

Rework the Preferences to prevent FOUC

New styles modules that is always added, so that all JS specific styling
is guaranteed to load before first paint. Reworked the HTML to generate
the preftoc (hidden when user has no JS).

Set htmlform nolabel class to use !important, so that it doesn't get
overriden by the 20% width rule of labels.

Also requires changes to the skinstyles of Vector preferences, which
is an a separate patch (I59f0f45), and other skins.

Bug: T115692
Change-Id: I24d9b16ed6729fdf0d59adcc2f0ba16f4f621b44