derivative: new application dedicated to derivative distributions

The initial implementation only displays a list of differences between
two distributions, but it's already useful to know which packages are
outdated/newer, etc.

[hertzog@debian.org:
- integrate cmpfunc in generatediff
- rename templates to use a "derivative" subdirectory
- fix the comparison view to properly return 404 for bad URLs
- add a title to the templates
- cleanup the tests to factorize code in add_package()
]

Signed-off-by: Raphaël Hertzog <hertzog@debian.org>