Move RazorPageGenerator SelectLists from GET handler to OnPageHandlerExecutionAsync

[AtOMiCNebula]

SelectList initializers are only being code-generated into the GET handlers, but not the POST handlers. Since .ViewData entries do not persist across responses, the page will not render correctly when the POST handler does not redirect (i.e. because validation failed).

This change moves the initialization into a shared OnPageHandlerExecutionAsync, which is invoked after model binding but before the handler itself. No need to dual-generate the initializers into both handlers!

[seancpeters]

@danroth27 @mkArtakMSFT - this is content fixes for the razor page scaffolding generated content.

[AtOMiCNebula]

FYI I've been unable to run the E2E_Test.RazorPageScaffolderTests collection of tests locally, and the ones I expect would consume my updated reference .txts appear to be being skipped in the Pipelines that get kicked off. I believe the .txts are correct, but still would like the chance to actually get the tests to pass before completing this.

Alternatively, I'll get the .nupkg installed locally and test the template generation by hand. Still hoping to get the tests themselves working first...I have an email going with Sean who suggested some things for me to try to get the local tests unblocked.