IServiceProvider.GetService now returns null for missing services

Previously GetService would throw if the requested service was missing.
This commit updates MVC to account for the change in behavior.
Replace calls to GetService<T> with GetRequiredService<T>

Even though GetService<T> still exists, GetRequiredService<T> preserves
the old behavior of throwing for missing services.
Split line made over 120 chars by call to GetRequiredService
Add `DefaultEditorTemplates.MultilineTemplate`
- #965
- test call-throughs from `Html.Editor[For]()` to inner `IHtmlHelper`
 - add another parameter to `DefaultTemplatesUtilities.GetHtmlHelper()`

nit: reorder dictionaries at the top of `TemplateRenderer` slightly
[Fixes #1332] Port IContentNegotiator to WebAPI shim for CoreCLR support

1. Ported IContentNegotiator.
2. Enabled Core CLR configuration
Addressed feedback
Enabled shim functional tests to run on aspnetcore50
Removed InvalidByteRangeException because there is no ByteRangeSteamContent on portable library