Hide Sequence Nav if only 1 item

@stvstnfrd & @jrbl - care to look this over so it can go out this week?
Do we have a policy for indentation in templates like this? Personally, I'd like to see the HTML indentation remain unchanged. Adding the leading space here:
- creates a mismatched indents in the rendered HTML
- more importantly, creates this big gnarly diff we see here

If you leave the indentation alone, the only thing that changes is
```
+ % if items_len > 1:
+ % endif
```

This is even more important since you're issuing the PR against origin. If this changes upstream, it's going to conflict _hard_.