Use 'Directory.Build.props' to make common Project Property consistent between projects.

[wu-yafeng]

Here are some common property in projects:

    <!-- Properties in PropertyGroup -->
    <Copyright>2015-2019 Ivaylo Kenov</Copyright>
    <VersionPrefix>3.1.1</VersionPrefix>
    <Authors>Ivaylo Kenov</Authors>
    <TargetFramework>netcoreapp3.1</TargetFramework>
    <NoWarn>$(NoWarn);CS1591</NoWarn>
    <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
    <GenerateDocumentationFile>true</GenerateDocumentationFile>
    <AssemblyOriginatorKeyFile>../../tools/Key.snk</AssemblyOriginatorKeyFile>
    <SignAssembly>true</SignAssembly>
    <PublicSign Condition=" '$(OS)' != 'Windows_NT' ">true</PublicSign>
    <PackageTags>aspnetcore;aspnetcoremvc;testing;unit;tests;fluent;testing;framework;asp;net;core;mvc;test;mymvc;mytested</PackageTags>
    <PackageIcon>nuget-logo.png</PackageIcon>
    <PackageProjectUrl>https://mytestedasp.net/</PackageProjectUrl>
    <PackageLicenseFile>LICENSE</PackageLicenseFile>
    <PackageRequireLicenseAcceptance>true</PackageRequireLicenseAcceptance>
    <RepositoryType>git</RepositoryType>
    <RepositoryUrl>https://github.com/ivaylokenov/MyTested.AspNetCore.Mvc</RepositoryUrl>
    <GenerateAssemblyProductAttribute>false</GenerateAssemblyProductAttribute>
    <RootNamespace>MyTested.AspNetCore.Mvc</RootNamespace>
    <IncludeSymbols>true</IncludeSymbols>
    <SymbolPackageFormat>snupkg</SymbolPackageFormat>

<!-- Items in ItemGroups -->

    <None Include="../../LICENSE" Pack="true" PackagePath="" />
    <None Include="../../tools/nuget-logo.png" Pack="true" PackagePath="" />

move those properties to 'Directory.Build.props' are much better.