Add support for custom footer

internal/dis/component/control/static/custom/template.go

@@ -3,6 +3,9 @@ package custom
 import (
 	_ "embed"
 	"html/template"
+	"text/template/parse"
+
+	"github.com/FAU-CDI/wisski-distillery/pkg/environment"
 )
 
 const footerName = "footer"
@@ -14,9 +17,26 @@ var defaultTemplate = template.Must(template.New("footer.html").Parse(defaultTem
 // Template creates a copy of template with shared template parts updated accordingly.
 // Any template using this should use one of the template contexts in this package.
 func (custom *Custom) Template(tpl *template.Template) *template.Template {
-	tree := defaultTemplate.Tree.Copy()
+	tree := custom.footerTemplate()
 
 	clone := template.Must(tpl.Clone())                 // create a clone of the template
 	template.Must(clone.AddParseTree(footerName, tree)) // add the parse tree to it
 	return clone                                        // and return the tree
 }
+
+// footerTemplate returns a new copy of the footer template
+func (custom *Custom) footerTemplate() *parse.Tree {
+	footer, err := (func() (*template.Template, error) {
+		data, err := environment.ReadFile(custom.Environment, custom.FooterPath())
+		if err != nil {
+			return nil, err
+		}
+		return template.New("footer.html").Parse(string(data))
+	})()
+
+	if err != nil {
+		return defaultTemplate.Tree.Copy()
+	}
+
+	return footer.Tree
+}