Fix dotnet/templating version branding flow

[dagood]

For example, auto-update to this 2.2 manifest should make templating produce something like 1.0.2-beta4-20180819-1962051. Ideally, identical.

Auto-update looks for a OfficialBuildId element to update (in this case to 20180820-01), but templating uses:

source-build/repos/templating.proj

Lines 6 to 8 in aadbd4a

	<BuildNumber>301</BuildNumber>
	<PackageDateTime>20170727</PackageDateTime>
	<PackageBuildQuality>beta2</PackageBuildQuality>

I think it'd be pretty easy to make a <OfficialBuildId>20180820-01</OfficialBuildId> and use a property function to chop it down to 20180820 for PackageDateTime. There isn't a clear way to update the BuildNumber property though.

Maybe we can rig up an updater to take the actual end result package version from the manifest and split it up into these properties.