[6.x] Sites & Site Groups

[riasvdv]

Notable changes:

• Split the Sites service into Sites and SiteGroups
• Moves the multisite check from Craft::$app->getIsMultiSite() to the Sites service, which can be used through the facade.
• craft.app.sites becomes craft.sites and craft.siteGroups in Twig templates

Additional changes not specifically for Sites & Site Groups:

• Added sessionErrors to global Twig to access Laravel session errors (to be used when controllers are ported)
• Added old() Twig function which accesses old input when validation errors occured to keep forms in the same state