UserSecrets causes design-time tools to crash

[natemcmaster]

Changes made to support MSBuild #525 attribute break design-time usages of .AddUserSecrets() because Assembly.GetEntryAssembly() is different at design-time than runtime.

Known design-time scenarios affected
EF migrations - see dotnet/efcore#6918
ASP.NET Scaffolding

Workaround:
Users should specify the app assembly with the overload of .AddUserSecrets(Assembly assembly)
Example

.AddUserSecrets(typeof(Startup).GetTypeInfo().Assembly)

[davidfowl]

This is pretty horrible...

[natemcmaster]

@davidfowl proposed mitigation: #544. Basically, we shouldn't make .AddUserSecrets() the default method anymore. IMO we should obsolete it.

[Eilon]

This patch fix is approved!