xenial (16.04 LTS) and bionic (18.04 LTS) test rebuilds

[Matthias Klose]

There are four test rebuilds done for xenial and bionic (main only), both the
release and the updates pockets.  The results of both builds are combined in one
table. Please see

http://people.canonical.com/~doko/ftbfs-report/test-rebuild-20180727-main-xenial.html

http://people.canonical.com/~doko/ftbfs-report/test-rebuild-20180730-bionic.html

The table also has some new color codes to mark packages which always failed to
build, and for packages which failed to build in a reference release.