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.)
```fix_taggings_count``` is only a virtual attribute, so I'm taking it out of the params hash that will be passed to the model, and using its value locally, later in the update action. If the checkbox was checked, a non-nil value will be present, so the counting of taggings will happen and be stored on the tag.