improve: better support of multi lang preview

[M4nouel]

Adding lang property in JSON schema allows to preview the different template translations according the value of this lang property.

But the default language mechanism was not support for the preview endpoint (/v1/templates/:name/preview). The PR adds support of multi language to the preview endpoint.

[stephane-klein]

@M4nouel suggestion: what do you think about keep the same "coding style" that https://github.com/spacefill/gibbon-mail/blob/6b4081d93cd3889b87437a1f12286899afebb2aa/backend/src/app.js#L198 ?

Why: to keep the code homogeneous 🤔.

[M4nouel]

Ok @stephane-klein I understand.

[stephane-klein]

Thanks @M4nouel