0.12dev: Always create a `site.html.sample` with instructions to activate it and a reference to TracInterfaceCustomization#SiteAppearance.
0.12dev: Always create a `site.html.sample` with instructions to activate it and a reference to TracInterfaceCustomization#SiteAppearance.

Closes #8869.

pkdevbox/trac,pkdevbox/trac,pkdevbox/trac,pkdevbox/trac