Open LibreOffice files in sd-svs-disp VMs

[emkll]

While LibreOffice is installed as part of #263, LibreOffice documents do not automatically open in the disposable VMs:

We should ensure LibreOffice files open automatically once they are send to an sd-svs-disp DispVM.

Originally uncovered as part of #263 (review) , relevant parts copy pasted below for convenience:

This is because xdg-open relies on the file extension and not magic numbers (file appears to rely on a file's magic numbers)

user@sd-svs-disp:$ xdg-mime query filetype 1-single-celled_authorship-doc 
application/zip
user@sd-svs-disp:$ file --mime-type 1-single-celled_authorship-doc 
1-single-celled_authorship-doc: application/vnd.openxmlformats-officedocument.wordprocessingml.document

Changing the extension does change the association:

user@sd-svs-disp:$ mv 1-single-celled_authorship-doc 1-single-celled_authorship-doc.doc
user@sd-svs-disp:$ xdg-mime query filetype 1-single-celled_authorship-doc.doc 
application/msword

We could either:
a) Write a wrapper script that receives the file from sd-svs, looks at magic numbers and opens it with the correct application, or
b) See if xdg-mime supports using only magic numbers (however preliminary research came back empty so far)
c) Add extensions to submissions on the SecureDrop server code

[eloquence]

See comment in #263 (comment) , is this moot as soon as we resolve freedomofpress/securedrop-client#163 ? If so that may be a better candidate for the next sprint.

[eloquence]

(Per chat with @emkll closing for now in favor of freedomofpress/securedrop-client#163 ; we can revisit if that is insufficient.)