Nouveau: Vertical nav toggle doesn't work in Customizer

[boonebgorges]

Vertical item nav styling (group, user) is currently controlled by *two* selectors: .bp-vertical-navs on the buddypress-wrap element, and .vertical on the nav element. This causes problems in the Customizer, because toggling 'Display the Group nav vertically...' triggers a refresh where only the .bp-vertical-navs class is present, not .vertical. As such, we have the following situation:

1. Your saved setting is for horizontal menus
2. You go into the Customizer and navigate to a group page
3. You toggle the Group Navigation to 'vertical'
4. The page refreshes, but the nav is still vertical in the refreshed frame.

This appears to be due to a bug in bp_nouveau_get_single_item_nav_classes(), where a strict check 1 === is being performed on a "temporary setting" that is never an integer.

[boonebgorges]

In 12044:

Nouveau: Ensure item nav classes are properly calculated in Customizer.

The strict === check was always failing because bp_nouveau_get_temporary_setting()
doesn't know when it should return an integer.

Fixes #7799.