Refactor html templates

This commit entirely refactors the use of html templates. Instead of
inheriting from a shared template, we insert the results into a base
template.

internal/dis/component/server/templating/template.go

@@ -0,0 +1,29 @@
+package templating
+
+import (
+	_ "embed"
+	"html/template"
+
+	"github.com/FAU-CDI/wisski-distillery/pkg/environment"
+)
+
+//go:embed "src/footer.html"
+var footerHTML string
+var footerTemplate = template.Must(template.New("footer.html").Parse(footerHTML))
+
+// GetCustomizable returns either a clone of dflt, or the overriden template with the same name.
+func (tpl *Templating) GetCustomizable(dflt *template.Template) *template.Template {
+	name := dflt.Name()
+
+	custom, err := (func() (*template.Template, error) {
+		data, err := environment.ReadFile(tpl.Environment, tpl.CustomAssetPath(name))
+		if err != nil {
+			return nil, err
+		}
+		return template.New(name).Parse(string(data))
+	})()
+	if err != nil {
+		return template.Must(dflt.Clone())
+	}
+	return custom
+}