Rework how symlinks work. The only directory to use is

Rework how symlinks work. The only directory to use is
/var/lib/roundcube. We use symlink from /usr/share/roundcube to
/var/lib/roundcube and not the other way. Moreover, plugins and skins
are also symlinked. A user should be able to add plugins and skins in
/var/lib/roundcube while default ones are in
/usr/share/roundcube. Closes: #612553.