Search returns total different than actual number of snaps

[Toto Bille]

Reported here: https://github.com/canonical-web-and-design/snapcraft.io/issues/2971

It seems that when searching for zsh the result gives 3 snaps but the total calculated of snaps is 4.

To reproduce:

```
curl -H 'X-Ubuntu-Store: ubuntu' "https://api.snapcraft.io/api/v1/snaps/search?q=zsh" | jq`
{
  "total": 4,
  "_embedded": {
    "clickindex:package": [{...},{...},{...}]
  },
  ...
}
```

[Daniel Manrique (roadmr)]

There's a "ghost snap" in the results for zsh, namely 'zsh-shell', this is a snap that had releases at some point but now has all its channels closed.

The list of 3 snaps is therefore accurate, those are the three that do have published releases. There might be a quirk in the logic to calculate the "total" value, because that is done on the list of snaps whose information did match the search criteria. So:

1- search for snaps matching "zsh" in the snap index, this returns 4 results.
2- When building the list of details for snaps, actually look at which ones have released revisions, and only 3 of the originally-found 4 do.
3- Calculate "total" based on the 4 results from #1, not the 3 from the properly-filtered list fom #2.

This probably needs fixing store-side, but note also that the "search" api is deprecated and you should be using the newer "find" api instead (http://api.snapcraft.io/docs/search.html#snaps_find).

[William Grant (wgrant)]

It's normal for searches to return approximate numbers. Consider, for example, what happens if a snaps leaves or enters the result set during navigation. I don't consider this to be a bug.