Allow loading stacktrace.js from a local asset

[stof]

Currently, stacktrace.js is loaded from a CDN (cdnjs by default). As the URL must be hardcoded in the config, using a local asset is harder as we cannot rely on the Asset component to handle the proper reference.

I see several reasons why we would want to use a local asset for the package rather than a CDN one:

• when deploying CSP, you might not want to whitelist cdnjs (which whitelists lots of of other libraries too) just for the error logging
• when writing software targetting enterprises, limiting the external dependencies makes things easier, as they regularly require whitelisting accessible domains (and so not requiring to whitelist cdnjs in addition to our own domain makes the process easier)

It would be great if there could be a built-in way to reference an asset through the Asset component as the source of stacktrace.js

[Seldaek]

If you want to PR this why not because I realistically won't have time for this any time soon.