Enable full framework support in core packages and templates

[SteveSandersonMS]

[SteveSandersonMS]

Status:

• Core packages now target netstandard2.0
• Samples target netcoreapp2.0;net461
• Templates by default target netcoreapp2.0 but support use of the TargetFrameworkOverride symbol to trigger generating net461 projects

Have not yet been able to verify that net461 works in any scenario. Need to wait for the universe build to make its next nightly drop so I can check against a consistent set of packages.

These changes are in both the 2.0.0-preview2 branch and dev currently.

[SteveSandersonMS]

The templates now generate working net461 apps as of commit 0526852