Feat: Add global instance CSS customization setting

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.

internal/validate/formvalidation.go

@@ -189,6 +189,16 @@ func CustomCSS(customCSS string) error {
 	return nil
 }
 
+func InstanceCustomCSS(customCSS string) error {
+
+	maximumCustomCSSLength := config.GetAccountsCustomCSSLength()
+	if length := len([]rune(customCSS)); length > maximumCustomCSSLength {
+		return fmt.Errorf("custom_css must be less than %d characters, but submitted custom_css was %d characters", maximumCustomCSSLength, length)
+	}
+
+	return nil
+}
+
 // EmojiShortcode just runs the given shortcode through the regular expression
 // for emoji shortcodes, to figure out whether it's a valid shortcode, ie., 2-30 characters,
 // a-zA-Z, numbers, and underscores.