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/assets.go

@@ -0,0 +1,22 @@
+package templating
+
+import (
+	"path/filepath"
+
+	"github.com/FAU-CDI/wisski-distillery/internal/dis/component"
+)
+
+// CustomAssetsPath is the path custom assets are stored at
+func (tpl *Templating) CustomAssetsPath() string {
+	return filepath.Join(tpl.Config.DeployRoot, "core", "assets")
+}
+
+func (tpl *Templating) CustomAssetPath(name string) string {
+	return filepath.Join(tpl.CustomAssetsPath(), name)
+}
+
+func (tpl *Templating) BackupName() string { return "custom" }
+
+func (tpl *Templating) Backup(context component.StagingContext) error {
+	return context.CopyDirectory("", tpl.CustomAssetsPath())
+}