Improve check for BP Classic Plugin

[samedwards]

Right now, checks for the BP classic plugin are done with a function_exists( 'bp_classic' ) conditional, which is checking for the existence of this function as defined in the BP Classic plugin. This relies on the alphabetical ordering of the active_plugins array (and of course the sorting of bp-classic before buddypress).

I ran across a situation where I was enabling the BP Classic plugin via a ​plugin manifest, which did not cause the active_plugins array to be sorted, and thus I got redefined function errors when BP Classic loaded after Buddypress.

A check to is_plugin_active() has the downside of requiring the BP Classic plugin to be installed in the correctly named folder, which is also not always a given, but I would suggest that the gating conditional being changed to function_exists() || is_plugin_active() should cover more possible circumstances.

[espellcaste]

@imath could you look into this?

[imath]

Absolutely, thanks for alerting me about it 👌

Trac ticket: ​https://buddypress.trac.wordpress.org/ticket/9181