"Upload" doesn't work with `make staticpkg`

Bug Template

Describe the bug
One method to distribute MakeCode is to build it into a package with make staticpkg. This renders everything to static html and javascript. However, the "Upload" button doesn't work, and reports "Compilation failed, please check your code for errors." even when using the default program.

To Reproduce
Steps to reproduce the behavior:

1. Clone pxt-chibitronics -- for our purposes I'm using one with custom C++ code at https://github.com/xobs/pxt-chibitronics/tree/rainbow-tape
2. Run make staticpkg
3. Serve the result with cd built\packaged; python -m http.server
4. Go to localhost:8000 and attempt to upload a program

Expected behavior
The program should build and upload.

Screenshots

Desktop (please complete the following information):

• OS: Windows 10
• Browser Firefox
• Version 3.3.4

Additional context
The output from staticpkg attempts to make calls to many pages that don't exist. For example, it does a POST to http://localhost:8000/compile/extension, which results in a 501 error. It also attempts to access http://localhost:8000/clientconfig which doesn't exist.

Start by running make staticpkg, serving built/packaged with python -m http.server, and reproducing the Upload failure. Trace the generated requests to /compile/extension and /clientconfig; done means the default program builds and uploads from the static package without requests to missing pages.