Files encryption

Proposed merge of the files_encryption branch. Master has already been merged into files_encryption several times.

files_encryption is not complete or in release state just yet. However Frank and I agree that it should be merged asap in order to avoid future divergence between these two long isolated branches.

files_encryption should have no impact on any other app or functionality, unless the it is enabled as an app.

Currently unit tests relating to the encryption app will not pass if executed via centralised /tests folder. For this reason unit tests are for now housed in apps/files_encryption/test instead of .../testS [capitalisation added]. The unit tests are important however and will pass when executed from within their own folder, so long as the app has been initialised.

Once it has been merged with master work will continue on improving it for its planned release with OC5.

Please review, suggested reviewers: @blizzz , @bartv2 ?