Rethink routing of projects/organizations

# Problem

Per @begedin in a PR:

> The current layout of /:slug/projects is strange and seems to be lacking. On the other hand `/organizations/:slug` seems the way to go.
>
> The thing is, having both seems a bit redundant. Is there any specific reasoning to have two, or did it just somehow happen along the way.
>
> Merging the "good" behavior from both places into a single namespace under /`:organization_slug` seems like the way to go.
>
> It also looks like we started off with `/:slugged_route_slug` with the impression of users being able to have projects at some point to. Is that still a goal? If not, I think we could rewrite our router and organize our files in a clearer, more understandable way, but it would involve a lot of changed files to do so.

This needs some discussion.