Some reorganization of activation code:

 - Make method name apply to trait implementor better (When a user agent is to run post-click activation steps on an element, it must run the activation behavior defined for that element)
 - Mention invariants and conditions on authentic_click_activation
Move InputSubmit to Activatable
Move InputCheckbox to Activatable
Refactor code for fetching radio siblings
Move InputRadio to Activatable