Notes:
* This is functional but intentionally not designed until the design
has been finalized.
* A feature flag is added that swaps out the limited width toggle with
a settings cog
* The preferences module is kept in a separate ResourceLoader module
(skins.vector.clientPreferences) to make it more portable for
when we eventually want to add it to Minerva as well.
* Intentionally have not focused on non-jS version given we will
likely revise this again in a future version/upstream to core.
Bug: T345363
Change-Id: I0c793b73e0676e32f031c6168c292f63908c72e3
