Fixed bug where `assemblePySB` was not producing downloads
Fixed bug where `assemblePySB` was not producing downloads

- `assemblePySB` has a second, optional parameter `export_format`. This function
   works as intended.
- the buttons in `base_script.js` which act to download models using promises
  chained to each other with `.then()` methods
- if a `then` method's output is passed to a function with multiple inputs, it will
  pass the response as the first, the status (here it was "success"), and an object
- this was passing `success` to the `export_format` parameter in the
  `assemblePySB` function.
- the function that requests the assembler was renamed `requestPySB` and is
  reused for all built models. The function that builds a model for download in
  the PySB format is now called `assemblePySB` and takes the form of functions
  like `assembleKappa`
pvtodorov/pathway_map_dynamic,pvtodorov/pathway_map_dynamic