Re-do site menus to make them more consistent

There are lots of inconsistencies in the Drupal menu structure on c.o. There so many small problems with the menus that I don't think it's even worth listing them all out. So instead, I pulled @nicw aside at the UK sprint this evening and pointed them out in realtime. He agreed that there are lots of problems but said that it would probably be a lot of work to re-do the menus because there are so many layers of hacks right now.

In any case, I made this google doc to propose one menu structure that we could use for the site:

https://docs.google.com/spreadsheets/d/1p_gIk4mWbAg6NimUWIj46MkT_PQcjmPN6b6PuoFIXAY/edit?usp=sharing

I'm hoping to have some more in-person conversations about this at the sprint this week.

CC @josh