Use the ext debug files for easier debugging.

Tested on regiogis, before:
```
Writing to regiogis/static/build/edit.js (1291 KB)
Writing to regiogis/static/build/app.js (1699 KB)
Writing to regiogis/static/build/xapi.js (1422 KB)
```
After:
```
Writing to regiogis/static/build/edit.js (1402 KB)
Writing to regiogis/static/build/app.js (1810 KB)
Writing to regiogis/static/build/xapi.js (1534 KB)
```
Than we have about 111 kB extra, I think that reasonable.