Add extra goodies to build report type

The build report structure gains two fields:

* A time stamp;

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

In addition, solver failure is added to the list of install outcomes.

This only changes the types 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
Fix build report text view

This should return a properly formatted report, not an internal dump.