migrate helpers.py from apps/amo (bug 1074184)

I removed the external_url filter here because it wasn't used anymore - the filter was only used in the reviewers template where I think its unneeded regardless.
If its removed as part of a commit that removes all the outgoing_url code then either this commit will be blocked on (controversial) outgoing_url commit; or the (now obsolete) filter function has to stick around - the last remaining function in helpers.py.