attachments (blobs) on private messages should be encrypted

[mixmix]

Currently, in private messages, attachments (blobs) are not encrypted (though the text is).

this PR disables patchwork attachments in private messages #538, but there should really be a solution added to patchcore

There's rumour patchfoo has a nice solution to this.

[mmckegg]

Here is patchfoo's solution: http://localhost:7718/%25YAg1hicat%2B2GELjE2QJzDwlAWcx0ML%2B1sXEdsWwvdt8%3D.sha256/commit/440274d19f302e0f293d56cba685e39e372cda12

(need to be running git-ssb-web or patchwork with git-ssb installed)