Add compatibility conversions for mobwrite

The new protocol is largely inspired by the original line-based
protocol, but has some improvements. This compatibility converter allows
people to convert from and back to the mobwrite protocol to ensure
compatiblity with standard mobwrite clients.
Update sohva and diffson versions

Sohva deprecated a lot of names and methods in its last version to avoid
confusion between kind of sessions. Change to new names in \BlueLaTeX
Merge remote-tracking branch 'upstream/master' into wip-web-client