Split each component's classes file, and move each individual class into its own file.

While historically manageable, the previous approach of having the majority of each component's classes in the same file is growing unwieldly with each new version of BuddyPress, and causing an avoidable increase in code complexity.

The existing -classes.php files are now a loading wrapper for the components' classes.

This change only affect classes that were explicitly declared within the -classes.php files. Any other classes, for example those in some components' template functions files, remain untouched for now.

Fixes #6083