4084 make draft deletion time more visible.

https://code.google.com/p/otwarchive/issues/detail?id=4084
Following discussion with the translation committee, we agreed that ts() strings should be interpolated with markup in them rather than split into little strings like this. So that would give us:

```ruby
<p class="caution notice"><%= ts("This draft will be <strong>automatically deleted</strong> on #{time_in_zone(work.created_at + 1.month)}.".html_safe) %></p>
```