siso: webui make invocation subpage tabs not top-level nav items

Now the top-level nav items make sense, because "Run Build",
"Invocations", and (Ninja) "Targets" are all siblings for an outdir.
Previously having the subpages top-level nav items meant there was a
mixture of nav items that were for specific builds, versus nav items
that were specific to the outdir.

The implementation is quite brittle but this is not a meaningful
downgrade from how siso webui has been implemented for its lifetime.

A followup CL will take care of the lack of route generators.

Bug: b/533258244
Change-Id: Ie044c31bfccd40c5f49ea1ce52f7e7b26a6a6964
Reviewed-on: https://chromium-review.googlesource.com/c/build/+/8139302
Commit-Queue: Richard Wang <richardwa@google.com>
Reviewed-by: Alex Ovsienko <ovsienko@google.com>