Remove slugs from bbPress class and rely on object cache

[johnjamesjacoby]

Originally bbPress loaded its slugs at run-time into class variables for safe keeping. Now that all slugs are reliable options in the database, we can remove these pointers and rely on the options and object caches to handle this.

This reduces code duplication, removes class variables that are no longer needed, and is a net performance and memory usage win.

[johnjamesjacoby]

(In [3760]) Remove slug variables from bbPress class and rely on options and object cache going forward.

• Net performance and memory usage win
• Removes class variables that are already in object cache
• Reduces code duplication
• Fixes #1760

[johnjamesjacoby]

(In [3761]) Use bbp_get_view_slug() in bbp_get_view_url(). See #1760.

[johnjamesjacoby]

(In [3762]) Introduce functions for getting the unique rewrite ID's for user/view/edit.

• Use new functions where needed
• Clean up variable usages in bbPress class
• See #1760