Merge lp://staging/~jtv/launchpad/bug-632880 into lp://staging/launchpad

Generate URLs straight from LibraryFileAlias.

= Bug 632880 =

This fixes a small and subtle problem in working with the Librarian. When we render links to persons, products, and so on, we include any icons that may have been set for them. The link formatter gets the LibraryFileAlias for each icon, and gets its URL.

Under the hood, the LibraryFileAlias delegates this work to the LibrarianDownloadClient. But instead of a reference to the LibraryFileAlias, it only passes an id. The LibrarianDownloadClient then looks up the same LibraryFileAlias in the database. This isn't so costly, except when the request is running off the master store and the LibraryFileAlias was retrieved from the slave store. In that case, all icon LibraryFileAliases get requested again. This is a waste.

In this branch I make retrieval of URLs work directly from the LibraryFileAlias object. This would be bad if there were a thread or process boundary between the point where the LibraryFileAlias is retrieved and where it's used, but that is not the case here.

A lot of the work is actually in making sure that the FakeLibrarian supports the new method, and that it stays equivalent to the one in the real Librarian.

To test:
{{{
./bin/test -vvc canonical.librarian.ftests.test_web
./bin/test -vvc lp.testing.tests.test_fakelibrarian
}}}

There's plenty of lint, I'm sorry to say, though none that I introduced. I'ld be happy to clean it up, but did not want to pollute the diff for the review.

Jeroen