Add a GenerateSuiteIndexes task
Part of #755 (closed).
Best reviewed by commit, although the commit that actually adds the task is still somewhat unavoidably huge; sorry about that. The trickiest bits were generating the Sources
and Packages
files (which I managed to squeeze into a single query each with some inspiration from dak, after some fighting with Django), and arranging for all the relevant collection items to end up with the right creation and removal timestamps so that serving snapshots can work easily later on.