Add random sort option for changeset listing

[rivermont]

I'm submitting a feature request

Brief Description

Under the available options for 'Order by' in the sorting menu and on the main page, add an option to randomly sort changesets.

What is the motivation / use case for this feature?

Allows better spot-checking of user/group/tag-filtered changesets, compared to checking changesets sequentially. Currently, one can sometimes accomplish this by sorting by something like 'object modified', but for very uniform sets of changesets this is ineffective.

[batpad]

I like this idea. @willemarcel do you have a sense of how much work this would be? Seems like the Django ORM supports this quite easily with .order_by('?') - I think my main concern would be if this has any significant performance drawbacks in the database.