Does not collect static files outside of static/

Originally filed as [293](Does not collect static files outside of static/) in django-simple-deploy, moved here as it's specific to Fly deployments.

I deployed a project to fly, and static files were missing. The project had some files in a root `css/` directory, which was in `STATICFILES_DIRS`. But in the fly-specific settings block, `STATICFILES_DIRS` is set to static/. It should have static/ appended if it's not already there? It should not write over whatever `STATICFILES_DIRS` was.

- [ ] Reconsider whether I need to define `STATICFILES_DIRS` at all.
- [ ] If keeping it, make sure to append to existing, if there is one. Don't overwrite user's setting.
- [ ] Check this on other platforms.
- [ ] Consider adding some custom css to the sample test project, to verify this behavior.
- [ ] Consider asking for some review of the approach to handling static files. A review of this platform's approach is probably good enough, then carry that approach to other platforms.