[pkg] various debian packaging fixes
- stop using *.install.in files, just use wildcards in *.install
- switch to X-Python-Version (preferred for dh_python2)
- also call dh_python2 for /usr/share/cubicweb, not just public modules
- drop long obsolete ginco and erudi migration code from cubicweb-ctl.postinst
- fix cubicweb-ctl cleanup: purge code belongs in postrm, not prerm
- don't compress changelog.html