Previously a Go package attempting to import Fleet packages would run
into an error like "server/kolide/emails.go:93:23: undefined: Asset".
This commit refactors bindata asset handling to allow importing Fleet as
a library without changing the typical developer experience.
- Add the server_url_prefix flag for configuring this functionality
- Add prefix handling to the server routes
- Refactor JS to use appropriate paths from modules
- Use JS template to get URL prefix into JS environment
- Update webpack config to support prefixing
Thanks to securityonion.net for sponsoring the development of this feature.
Closes#1661
