BuddyPress 12.0 compat

[boonebgorges]

See boonebgorges/buddypress-group-email-subscription#242, boonebgorges/buddypress-docs#727, boonebgorges/invite-anyone#176 for discussion and prior art.

Note that we might be able to get away with not having a < 12.0 compatibility layer, because the BP version number is controlled by CBOX.

[r-a-y]

For bbPress, see https://bbpress.trac.wordpress.org/ticket/3576 .

Might we have to consider adding the BP Classic plugin?

[boonebgorges]

Good point. We'll probably have to include BP Classic at least for the plugins we don't control.

I still think we should work in our own codebase toward phasing out legacy URL concatenation in favor of BP 12.0, because it will give admins a better experience with slugs, etc.

[boonebgorges]

Given that we are going to be releasing in early January, just a few weeks after the release of BP 12.0, I think we should hold off on bumping our BP version past the 11.x series. We'll take a bit more time to update our internals to use BP 12.0's native functions, and also give BP a chance to shake out any issues after the 12.0 release.