Cleanup/prepare xaml pages for additional changes

[krschau]

Summary of the pull request

In preparation for larger changes that are going into the Experimental Features page, Environments Landing page, and SetupFlow MainPage, this PR does some rearranging, cleanup, and whitespace changes. The only functional change is adding a margin to the SetupFlow main page so that each set of SettingsCards is the same distance underneath its header.

Detailed changes broken down by page:

• ExperimentalFeaturesPage.xaml: whitespace, remove VerticalScrollBarVisibility which was set to the defau.t value anyway.
• LandingPageViewModel/LandingPage: instead of using the Loaded event, use i:Interaction.Behaviors to call ViewModel's LoadedCommand when the page is loaded.
• LandingPage.xaml: whitespace
• MainPageView.xaml: whitespace, fix indentation, simplify Uid references.

References and relevant issues

Validation steps performed

Validated locally.

PR checklist

• Closes #xxx
• Tests added/passed
• Documentation updated