Convert sd-journalist to sd-proxy

[conorsch]

Porting the discussion from #107 (comment):

We confirmed that a new sd-proxy VM (which takes the place of the sd-journalist VM) should expose a qrexec service called by sd-svs.

It's not just a nomenclature change, it's a full tooling swap. We'll need shake out the sd-proxy config (and packaging story) separately, which will unblock this work.

[eloquence]

Edited top-level task since #127 was resolved a while ago. To recap, the key remaining blocking tasks are #156, #131 (will be closed by #160) and #161.