Fix namespace templating inconsistencies (#713)

### Description of the change

Added namespaces to all the template files to better support alternate templaters in gitops systems

### Benefits

Gitops system that have different ways of handling helm templates can actually deploy this chart correct, especially through subcharts

### Possible drawbacks

Potential regression when upgrading, though this should be unlikely per @jessesanford 's comments with it defaulting back to the existing behaviour

### Applicable issues

  - Addresses https://gitea.com/gitea/helm-chart/issues/630
  - Addresses https://gitea.com/gitea/helm-chart/issues/557
  - Addresses https://gitea.com/gitea/helm-chart/issues/623

### Checklist

- [X] Parameters are documented in the `values.yaml` and added to the `README.md` using [readme-generator-for-helm](https://github.com/bitnami-labs/readme-generator-for-helm)
- [X] Breaking changes are documented in the `README.md`

Co-authored-by: SorsOps <80043879+sorsOps@users.noreply.github.com>
Reviewed-on: https://gitea.com/gitea/helm-chart/pulls/713
Reviewed-by: pat-s <pat-s@noreply.gitea.com>
Reviewed-by: justusbunsi <justusbunsi@noreply.gitea.com>
Co-authored-by: SorsOps <sorsops@noreply.gitea.com>
Co-committed-by: SorsOps <sorsops@noreply.gitea.com>

values.yaml

@@ -20,6 +20,9 @@ global:
   #   hostnames:
   #   - example.com
 
+## @param namespace An explicit namespace to deploy gitea into. Defaults to the release namespace if not specified
+namespace: ""
+
 ## @param replicaCount number of replicas for the deployment
 replicaCount: 1