Update template links to be relative paths instead of absolute paths

[danroth27]

The nav links and fetch data URL in the templates are all absolute paths with a leading slash, which means they don't honor the base path for the app. I think these paths should be relative paths (no leading slashes).

[danroth27]

One potentially related issue is that a customer reported that if they change the nav link for the home page to be the empty string (href="") then they get a compilation error with the 0.4.0 dev builds:

Error CS1525 Invalid expression term ')' WebApplication3 C:\Users\xxx\source\repos\WebApplication3\WebApplication3\obj\Debug\netstandard2.0\Shared\NavMenu.g.cs

builder.AddAttribute(29, "href", );

An empty string is a valid relative URI. I don't see this issue with 0.3.0.

The customer was able to workaround this issue by using href="."

[SteveSandersonMS]

Fixed with dotnet/blazor#878.