Nouveau should check the component is active before trying to use a `bp_get_{component}_slug()` function

[imath]

This is a follow up of #8133. [12892] introduced direct uses of bp_get_{component}_slug() functions (without checking the corresponding component is active). If the corresponding component is not active it can generate fatal errors as the corresponding bp_get_{component}_slug() function is not available.

[imath]

In 12908:

BP Nouveau: introduce a function to get active component slugs

bp_nouveau_get_component_slug() accepts the component's ID as an argument and uses the bp_get_{$component_id}_slug() corresponding function to get its slug only if the component is active.

Replace all occurences of bp_get_{$component_id}_slug() by bp_nouveau_get_component_slug( $component_id ) to prevent errors.

Fixes #8464