Multisite subdir install

[michaelw85]

• [x] I've read the guidelines for Contributing to Roots Projects
• [x] This request isn't a duplicate of an existing request
• [x] This is not a personal support request that should be posted on the Roots Discourse community

Summary

Add support for multisite installs with sub dir install instead of subdomain. e.g.

• domain.com (primary site, "site1")
• domain.com/site2

Motivation

This issue seems to have quite some history and seems to be fixable using (Nginx) rewrite rules looking at https://discourse.roots.io/t/working-subdirectory-multisite-nginx-rules/1503

I'm probably not the only one that had to spend quite some time trying to figure out how to get this working with Bedrock. Having this within the multisite URL fixer (if possible) would save quite some headache.

Additional context

This feels more like a bug to me than a feature request. Since it's fixable with rewrite rules I thought it to be an enhancement. With the rewrite rules in place, I now wonder if this plugin is needed at all.