RBB-generated js should be kept separate from project's js

Currently all JavaScript files are stored in the web-ui_exported/js directory, but it should be possible to blow away the whole web-ui_exported directory and regenerate it from RBB without losing anything. Therefore, any project JavaScript files (e.g., cryptex.js) need to live in another directory outside of the web-ui_exported directory. Beyond the mechanics of moving the files, the function send_js() in server.py will need to be updated to support searching in two different locations for js files.

Start with server.py and inspect send_js(), then trace how files under web-ui_exported/js are generated and served. Separate project JavaScript such as cryptex.js from the generated directory while preserving regeneration, and verify that send_js() finds files in both locations without losing project files.

Written by the indexing model from the issue text.