Implement "atpath" asset endpoint

[jjnesbitt]

Closes #2172

This implements the design doc laid out in #2155 (link).

There are some additional changes included in this PR, that may seem unrelated, but were necessary for this feature's functionality. One such change is the slight restructuring of urls.py, which was done in order to mount two separate swagger pages, with their own respective visible endpoints. This is actually an overall code quality enhancement, as before, our URL and swagger configuration was very ad hoc.

@jwodder The only deviation from that design doc is that the endpoint is instead hosted at api/webdav/assets/atpath, with dandiset_id and version_id provided as query arguments. This is done to separate concerns from the public API, and what's needed for webdav. Let us know if you find any issues.

[jwodder]

api/webdav/assets/atpath

I'm not really a fan of this path, not least because I don't anticipate many dandidav-specific routes being added. What about changing webdav to internal?

[jjnesbitt]

I see there is a single test failure, which seems unrelated, but I'll investigate.

[jjnesbitt]

api/webdav/assets/atpath

I'm not really a fan of this path, not least because I don't anticipate many dandidav-specific routes being added. What about changing webdav to internal?

We discussed this internally and decided that the /internal prefix is essentially redundant, since we'll want to namespace any "internal" APIs, to keep things organized. Keeping the /webdav prefix, even though it may only ever be one endpoint, allows us to separate concerns in a meaningful way.