.NET Core 3.0 - Githubissues

Changes in all projects

changed target framework to .NET Core 3.0
upgraded NuGet packages
changed Program.cs to .NET Core 3.0 version
replaced Kros.AspNetCore extension method AddApi in ConfigureServices with .NET Core 3.0 method AddControllers
moved ApplicationInsights initialization to ConfigureServices (Microsoft migration recommendation)
added UseRouting method to Configure and replaced UseMvc method with UseEndpoints alternative:
```
app.UseEndpoints(endpoints =>
{
endpoints.MapControllers();
});
```

added service collection extension for setting proxy to HttpClient (in new version of HttpClient bypassing local adresses is broken, so in this case proxy must be set explicitly)

added global proxy for HttpClient (because of authorization middleware)
moved ServiceCollectionExtensions to Application folder, so it follows our architecture guidelines
removed ClaimsPrincipalExtensions, as this class was already copied to Kros.ToDos.Base

added global proxy for HttpClient (because of request for sending user role when creating company)