[Merge] lp:~abreu-alexandre/webbrowser-app/content-hub-pdf into lp:webbrowser-app

[Michael Sheldon]

Review: Approve

Looks good, I have some concerns about the Document type as a whole since not all apps can handle all types of Document, but we'll just have to live with that until mimetype support is implemented in content-hub (plus apps will need to be able to cope with receiving files they can't read anyway in case an exporter mislabels something.)
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https://code.launchpad.net/~abreu-alexandre/webbrowser-app/content-hub-pdf/+merge/236959
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