Issue 2221 redux manually reset taggings count

I think for this one it's actually worth reopening:
https://code.google.com/p/otwarchive/issues/detail?id=2221
because it was never implemented as requested before.

Now it is, except that instead of a button there's a ticky box, like @sarken suggested.

Also fixes the wide unicomma for bookmark tags, because hey, while I was in there. (I was trying to reproduce the steps for the taggings_count bug, which would have involved editing of bookmarks, so I came to examine the bookmark model where the tags get created.)
Just a few little things! Can you add a legend and heading to this? It won't do anything the the visual layout, but it'll help make it easier for screen reader users to jump around on the form. This should be fine:
```
<legend><%= ts('Taggings Count') %></legend>
<h3 class="landmark heading"><%= ts('Taggings Count') %></h3>
```

And the note on line 92 about what the checkbox does should be in a `<p>` tag, like the other notes. (The resulting lower-than-the-checkbox positioning will make it clearer it's not the checkbox's label, so clicking it won't select the checkbox -- we ran into some confusion on that once before. In the future, we'll find better ways of styling instructions and explanatory notes on forms.)