Fix subsite-to-subsite dispatch

[AriFordsham]

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Fixes #1803 .

It is now behaving correctly on https://github.com/jezen/subsite-with-static (see https://github.com/AriFordsham/subsite-with-static/tree/ari/yesod-change).

It's a bit messier than I wuld like, as well as being a hack overall - see #1803 (comment). The type signature to subTopDispatch makes sure this builds when mkDispatchInstance passes subHelper to it (even though subHelper is unused ☹)

[snoyberg]

I can't really claim a lot of familiarity with this code anymore, it's been a while, but the overall approach seems valid. I'd recommend including a test case to demonstrate that this is working. And if all the existing test cases pass, I think that's a good sign that it's working as expected.

[AriFordsham]

A fails-before, passes-after test is hard to write, because the failure is compile-time. I guess I could just include a simple test which show subsite-to-subsite dispatch working.

[jezen]

I think that sounds sensible 🙂

[AriFordsham]

Added a test case. This does not compile on master, and passes on this PR.

[snoyberg]

Thank you!