feat(router): auxiliary (aka "double slash") routes

[btford]

No description provided.

[naomiblack]

punted back to backlog as a few other router issues more urgent.

[btford]

I have some work done on this, but not enough for a PR. I'm going to punt this to next week to prioritize bug fixes. @matanlurey has encountered some problems that he's going to file issues for.

[btford]

I made some headway on this last week.

I just need to re-implement redirects, fix a few broken tests, and add tests for more complex use cases.

There will need to be a follow-up feature to make it possible to deep-link to aux routes.

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