Encrypt exported tokens file

[davideroffo]

With this PR #590 the export and import feature has been added to the application.

Currently, we are storing a plain text file with the file content (information about secrets) easily readable and importable from any user.

We want to store the exported file with encryption, where the secret should be a concatenation of [secret_string].[user_id]
In this way, the file is no longer readable and can be imported only by the user (user_id) who created (exported) that file.

Things to do:

• while exporting encrypt the file-content with [secret_string].[user_id] secret
• update the import method in order to decrypt first the file-content the user is trying to import
• in order to be backward compatible, if a file is in plain text import it anyway for now (@simoneb is this ok?)