Move all common settings files to `sites/all/settings` about blt

[danepowell]

Re-posting from #141...

Your argument makes complete sense and I was initially inclined to agree. However, at least within PS, it's rare that we launch new multisite projects on ACE any more. Multisite almost always goes on ACSF, which doesn't use either all or default anyway. So to me it actually makes sense to keep using default, since that aligns with the most common use case. It's really a pain on a single-site installation when some config exists in sites/all and other config in sites/default.

I think you could make a good argument either way. Regardless of the merits of all vs default though: we've already flipped back and forth between them at least twice, and the transition sucks for anyone already using Bolt. I don't particularly want to do it again unless there's a pretty compelling reason.

[danepowell]

Not sure if it's worth the added complexity, but we could have a "multisite" switch in project.yml. If a project is multisite, settings go into sites/all. Otherwise they go into sites/default.

[greylabel]

The compelling reason, IMHO, is it is better architecture to treat default the same as you would any other site in a multisite and not a special, global snowflake. For single-site installations, I don't really know how much heartburn this will cause if BLT handles the provisioning and is well documented -- developers should already be comfortable with the ideas of using sites/all with single-sites for modules and themes, not much difference here in concept.

That said, I really like the idea of "multisite" switch in project.yml or better multisite considerations throughout the project, including build.yml, box/config.yml, CI integration, and all other config files.

[grasmash]

This is now a moot point as of 8.4.0, given that default settings are now in vendor/acquia/blt/settings. Only local settings are insites/default and these are site specific.