Don't use URLify if a user has manually set a slug

Not a big deal, and can easily be handled by using the available js_hook,
but this has annoyed the hell out of me and others may find it useful.
Merge branch 'master' of https://github.com/benemery/wagtail into benemery-master
add Ben Emery to contributors
Translation to Portuguese Brazil
Fix message moderation
Delete temporary file
Fix python formats and compile messages
Insert responsible of translation in Portuguese Brazil
keep translations in alpha order
Prevent DB lookups with local cache of filters
changelog updates
Merge remote-tracking branch 'origin/master' into search-changes