Enable admin user password creation/update mode in values (#677)

### Description of the change

This enables sane modes for forcing reset, as well as providing more options to users of the chart by giving them the flexibility to set the mode for password creation/modification as part of init whether the user exists or not.

### Benefits

The new default should revert to the behavior before #673 became an issue, while also providing more flexibility for users who want to be able to manage their initial admin user password out-of-band after creating it the first time.

### Possible drawbacks

None that I can think of.

### Applicable issues

- fixes #673

### Additional information

See the discussion in #675 as well

### 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)

Reviewed-on: https://gitea.com/gitea/helm-chart/pulls/677
Reviewed-by: justusbunsi <justusbunsi@noreply.gitea.com>
Co-authored-by: James Harmison <jharmison@gmail.com>
Co-committed-by: James Harmison <jharmison@gmail.com>

templates/gitea/deployment.yaml

@@ -243,6 +243,8 @@ spec:
             - name: GITEA_ADMIN_PASSWORD
               value: {{ .Values.gitea.admin.password | quote }}
             {{- end }}
+            - name: GITEA_ADMIN_PASSWORD_MODE
+              value: {{ include "gitea.admin.passwordMode" $ }}
             {{- if .Values.deployment.env }}
             {{- toYaml .Values.deployment.env | nindent 12 }}
             {{- end }}