internationalize templates

• when requesting the email doi template from Alice (dApp in send-mode) (get-doi-mail-data.js)
• when visiting redirect url (in correct language)

What we need is the possibility to add a optional lang and optional template param to the REST-API of the send dApp which selects language and template param which you can edit in the send dApp. There it should be possible to add new template where each template has a default language and 'n' translations. The templates and translations should be saved in the local db.

Confirm-dApp is then getting the right template and translation for that user.

Also see: https://github.com/Doichain/dapp/issues/28