Rename abilities to correspond contoller/model action names

write_ was renamed to create_
modify_  was renamed to update_

So now in update action we have next code

def create
  can?(current_user, :create_issue, @issue)
end

def update
  can?(current_user, :update_issue, @issue)
end

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>