Replace pre-alpha workflow with alpha client

[eloquence]

Once the following dependencies are met, we should replace the pre-alpha workflow with the alpha client:

• Download and decrypt messages - Download and decrypt messages securedrop-client#40
• Decrypt downloaded documents - Add support for decrypting downloaded documents securedrop-client#66
• View downloaded documents - Add support for viewing downloaded documents securedrop-client#20
• Create updated client package - Create Debian package securedrop-client#22

Stretch for MVP integration:
Replies to sources - freedomofpress/securedrop-client#16

We will need the securedrop-client package to be updated and delete existing code for the pre-alpha workflow, update the README, etc.

As part of this work, we should convert sd-journalist to sd-proxy (#138). (let's do as a separate issue)

This will enable us to QA on master with the most important client functionality. Deletion (freedomofpress/securedrop-client#18) can be handled as an iteration.

[redshiftzero]

packages that should be installed in sd-svs: securedrop-client, python3-pyqt5, python3-pyqt5.qtsvg