[web] Stop using negative indices with os.stat results
The python doc says:
More items may be added at the end by some implementations.
.st_mtime is also more readable than [-2].
Document: cubicweb-doc
Title: CubicWeb documentation
Author: Logilab
Abstract: Some base documentation for CubicWeb users and developpers
Section: Apps/Programming
Format: HTML
Index: /usr/share/doc/cubicweb-documentation/index.html
Files: /usr/share/doc/cubicweb-documentation/*.html