Add local backend mode

[adamnovak]

This revises the API setup and lets us toggle to a new in-browser backend implementation that uses a WebAssembly WASI build of the GBZ-Base Rust implementation of GBZ graphs as SQLite databases.

You can "upload" (really grant access to) a file and then jump anywhere in an indexed path in the graph instantly, but the local backend still can't list paths or do reads, or usefully link to locations (since the file accesses don't get retained the next time you open the tube map.) It also can't read BED files yet.

Here's the new configuration at the bottom of the customization accordion:

You can Extract tube map data: either On remote server or In-browser (.gbz.db uploads only!).