Introduces the BuddyPress Attachments API

BP_Attachment is a new class custom and core components can extend to manage file uploads.

• It is taking care of setting the uploads data (path, url..)
• It is creating the specific base uploads directory for the component if requested
• It is taking care of uploading the files into the uploads directory
• Custom validation rules can be set in order to for instance: restrict mime types or file size
• If, like avatars, cropping an image file is needed, the class includes a crop method to help you.

bp_upload_dir() is also a new function to make sure the uploads data will always be the one of the site BuddyPress is activated on (the root site).

Props johnjamesjacoby, boonebgorges

See #6278