Add custom scripting

[jeremystretch]

Closes: #3415

Introduces the ability to run arbitrary custom scripts with data provided via the NetBox UI

[lampwins]

I am really liking this. In my testing I found a couple of things:

• We should gracefully handle a script failing to import. Currently, the dynamic import logic has no error handling, which results in generic 500 responses. E.g., failing to from extras.scripts import Script :)
• Changelog entries (and presumably webhooks) are being generated regardless of the DB transaction commit state. I fear this will not be easy to overcome, given our fancy object change middleware.
• In the spirit of the changelog, I wonder if it would be worth creating a new type of change in the changelog to allow logging that a script was run. This would allow the changelog request ID to be tracked back to a script execution event. Given my thoughts on the previous item, I am not sure how feasible this would be to implement.
• Several of the models that users are likely to interact with use slug fields, so I was thinking it might be nice to add a slug boolean argument to the StringVar that uses a prebuilt and consistent regex value.
• Along those same lines, we may also want to support a JSONVar to make handling of potential Config Context data easier.
• I noticed the ObjectVar does not use the Select2 widget. This will surely become a pain point for users, but I admit the logic to make this happen is not likely to be trivial.
• I also see a use case for a static choice variable type. This can be used to mimic static choice fields in NetBox core, but also for use case specific choice limiting.
• I find the UI to be a bit inconsistent due to the fact we use bootstrap pannels for all other forms.

In all though, I am very excited about the prospects of this feature. Well done @jeremystretch!

[jeremystretch]

Changelog entries (and presumably webhooks) are being generated regardless of the DB transaction commit state.

Good catch! This should be fixed in 9a9660a. I've introduced a new signal that can be used to purge cached changes to be logged. It's still a bit hacky at the moment, since deletion ObjectChanges are currently written immediately rather than cached, but I think I have a better approach for that as well (out of scope for this PR).