This is a very rough (but working and happy-dance-inducing) patch that adds a
mass-update or batch-edit interface, along with underlying plumbing to make it work.

I'll try to post a few screenshots before Monday.

Entry point is dependent on issue 246 (which needs some love too). No
confirmation dialog is displayed, but the supporting parts are in place. The
template and code need some cleaning up, tests and docs.  It still lacks success
feedback (failure feedback is rough, but exists). No security checks are
implemented so far.

With so many open issues, one might question why post this at all. My only
answer to that would be yet another happy-dance :D

Slightly improved version. Still lacking docs, confirmation dialogue and some
permission checks (only to avoid misleading users).

Added:
    * Success and (better) error messages.
    * Entry point from index view.
    * Option to not unset Links ('no change').
    * Nice view of edited issues on success/error.

Changed/fixed:
    * Avoid 'non-indexable' errors when the 'id' form value isn't a list.
    * Reject batch edits without issue IDs
    * Disable the comment field for now

Demo and review:
http://tinyurl.com/batch-edit (looks much better when logged in :)
http://codereview.appspot.com/40130

Screenshots :)

Fix a problem with the results page not showing issues.