Mail Download: Convert LF to CLRF

With the GEVER Release 2.9 it's possible to directly download the *.eml File. But sometimes the mails are displayed faulty in MS Outlook. 

We find out that converting the line endings from `LF` to `CRLF ` solves the problem. Therefore I've implemented a separate `download` view for mails, which converts the line endings before delivering the data.

@lukasgraf please have a look ...