Page move locale constraint 931156

Superseding #1598, with changes that already got a quick looking at:
```
<ubernostrum> lorchard: it looks good to me.
<ubernostrum> lorchard: only comment is that since you have access to the doc, it's simpler to just do the filtering on self.locale.
```
Took care of that comment, going to see what jenkins thinks of this PR, and probably just merge & push to staging for writing team to play with.