[feature] Add global instance CSS customization setting (#3352)

Allow instance admins to add custom CSS that will affect
every page of their instance.

This is done with a new CustomCSS instance setting that
works pretty much exactly like the Users CustomCSS property.
This custom CSS is then requested for every page load.
User styles/themes take precedence over this CSS.

Co-authored-by: tobi <tobi.smethurst@protonmail.com>

internal/api/model/instancev2.go

@@ -53,6 +53,8 @@ type InstanceV2 struct {
 	Description string `json:"description"`
 	// Raw (unparsed) version of description.
 	DescriptionText string `json:"description_text,omitempty"`
+	// Instance Custom Css
+	CustomCSS string `json:"custom_css,omitempty"`
 	// Basic anonymous usage data for this instance.
 	Usage InstanceV2Usage `json:"usage"`
 	// An image used to represent this instance.