server: redirection rework: - redirection parsing is now launched after LiberviaPages are imported, this way the pages are accessible and their methods are usable - inv_redirections attribute in LiberviaRootResource keep a map from new URL to old URL, allowing to check if a redirected path can lead to the same page - LiberviaPage methods are used to retrieve their URL - fixed remaining args when going to a redirected page - new checkRedirection method to see if a redirection can lead to the same URL

try:
    from common.constants import Const as C
except ImportError:
    pass # import doesn't work at this stage with pyjamas
else:
    __version__ = C.APP_VERSION