squashme: gitian: fix qt determinism

Fixes the 'date' wrapper which wasn't actually functional. Sequential builds
would be the same, but builds crossing a date threshold would not.
squashme: gitian: move osx qt to 5.2.0 to match windows

TODO: Bump them both to 5.2.1 simultaneously
squashme: docs: shuffle around the osx documentation a bit

Added the OSX gitian steps to the release process docs.
The checksums and signing info has not been added because these are likely
to change a few more times before merge.

TODO: fill in checksums and signing info post-merge