Add build log retrieval to reports feature API
Implement build reports view
Give tables a bit of wiggle room
Add extra fields to build report type

Two fields are added:

* A time stamp;

* Documentation builder flag, which is True iff the client was
  generating documentation for upload.

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
Add queryPackageReports
Add build status indicator to package front page
Only allow doc builder flag when user can upload docs
Build client: add doc builder flag to submitted reports