Implement build reports
Add a timestamp to the build report type

This only changes the type and migration machinery; front-end support
will come in a later commit.

The existing migration code (commit 111f3897) has also been removed; I
couldn't make it fit with the new changes. As the former is quite old
(Nov 2013), the removal shouldn't be too worrying.
Add timestamp to build report view
Affix a timestamp to every submitted report