Administration: add a new settings tab to manage slugs customization

Compared to [13442], change the logic of Components user navigation
generation by introducing a BP_Component::register_nav() method to
globalize the nav items early (ie: the registration step) and make them
available for the new settings tab to manage slugs customization.

After a second thought, the BP_Component::setup_nav() should remain the
navigation generation step instead of playing the registration role. This
will maximize backward compatibility & third party plugins wishing their
slugs to be customizable will need to "opt-in" for BP Rewrites using the
BP_Component::register_nav() method.

This first version of the URLs settings tab does not handle slugs
customization yet, its first usage is to make sure all BP Components user
navigation slugs were registered & to put the Accordion UI in place.

Props r-a-y, johnjamesjacoby, boonebgorges

Closes ​https://github.com/buddypress/buddypress/pull/84
See #4954