add paper synchronization Rest API
add Rest service to return session data
add Rest service to backup the paper sources
add Rest service for listing synchronized files

a paper consists in several source files. Synchronized files are the
\LaTeX related files (*.tex, *.bib, ...). These files can be open in a
real-time synchronized editor
add service for listing non synchronized files
make paper conf available in `RoleLet`

an HLet that works with a paper is most likely to use the paper
configuration, hence it might seem meaningful to make it availble
instead of redefining it in all implementation classes
add service to delete paper