Project restructuring to simplify dev, especially multi-platform dev

[fluffynuts]

Some projects haven't received updates in a very long time - simply because I'm not using them.
Some of these aren't even on nuget.org, eg:

• PeanutButter.MVC
• PeanutButter.NetUtils

Other projects are on nuget.org, but are unlikely to receive functional updates because I'm not using them, but I'll support issues, if anyone brings them up. In particular, I'd want to target packages which depend on other nuget packages (thus which should have maintenance updates for dependencies) or packages where the same functionality can be found in actively-maintained packages

• PeanutButter.Utils.Entity (I don't use EF, and even if I did, I'd probably use EFCore, not EF itself - deps are way outdated on this)
• PeanutButter.TestUtils.Generic (NExpect does a better job)

In addition, PeantuButter.ServiceShell and PeanutButter.WindowsServiceManagement should probably move off into their own area as they can't build or test on anything but Windows (which is their intended target) and they are relatively stable - so they hold back being able to work on PB on Linux for no real benefit.

There may be others, but the idea should be to make build, test & publish possible cross-platform (this would also require updating to use dotnet to build and push nuget packages - see #53)