DRY improvements (#664)

### Description of the change

Adding support for DRY principle (via use of the TPL function) to the PVC storage class and the ingress class

### Benefits

It allows to reference a variable into another one to avoid duplicating them (or using YAML anchors).
It is useful and valuable when including Gitea into an umbrella chart with multiple components and to have a single variable while the components variable only refer to that single one.

Example 1
```
global:
  persistence:
    storageClass: "storage-class"
persistence:
  storageClass: "{{ .Values.global.persistence.storageClass }}"
```
This results in having `spec.storageClassName` equal to `storage-class` in the PVC object

Example 2
```
global:
  ingress:
    className: "ingress-class"
ingress:
  className: "{{ .Values.global.ingress.className}}"
```
This results in having `spec.ingressClassName` equal to `ingress-class` in the Ingress object

### Possible drawbacks

N/A

### Checklist

- [X] Templating unittests are added

Co-authored-by: 212597596 <cedric.henry@ge.com>
Co-authored-by: pat-s <pat-s@noreply.gitea.com>
Co-authored-by: justusbunsi <justusbunsi@noreply.gitea.com>
Reviewed-on: https://gitea.com/gitea/helm-chart/pulls/664
Reviewed-by: pat-s <pat-s@noreply.gitea.com>
Co-authored-by: Ceddaerrix <ceddaerrix@noreply.gitea.com>
Co-committed-by: Ceddaerrix <ceddaerrix@noreply.gitea.com>

templates/_helpers.tpl

@@ -74,7 +74,7 @@ imagePullSecrets:
 Storage Class
 */}}
 {{- define "gitea.persistence.storageClass" -}}
-{{- $storageClass := .Values.persistence.storageClass | default .Values.global.storageClass }}
+{{- $storageClass :=  (tpl ( default "" .Values.persistence.storageClass) .) | default (tpl ( default "" .Values.global.storageClass) .) }}
 {{- if $storageClass }}
 storageClassName: {{ $storageClass | quote }}
 {{- end }}