First pass at bp-notifications component. Includes:

• Backwards compatibility for old core functions.
• Screens, functions, classes, and actions for Notifications.
• Improved class methods for getting, updating, and deleting notifications.
• Template parts in bp-legacy for theme compatibility.
• A few basic unit tests.

@todo's:

• Improve template output with dedicated functions, markup, classes, et all.
• More unit tests.
• Pagination links.
• Auto-activate on update, so existing installations do not lose previously core functionality.

See #5148. Props johnjamesjacoby, boonebgorges, r-a-y.