Fix width class missing if KBIN_PAGE_WIDTH has not been set

[BentiGorlich]

When the page width has not been explicitly set the width--fixed class was missing from the kbin contianer. That lead to the subscription sidebar not having a max width leading to this display:

[e-five256]

It'd also be doable by removing width--fixed here as that's how the main container is styled (the lack of class means not the other other two) but this also works. Perhaps it's better to always be consistent with having a class name

(I think the thought was if someone had a malformed setting or deleted the setting for some reason it would fall back to the default, this happens for themes too, if I go delete the theme out of the class list it's not unstyled, it just falls back to the light theme. So removing it might be better because if we were to ever remove a setting, as unlikely as that might be, at least it will still have styles rather than none)

[melroy89]

Ow so that was happening to me randomly!