Create new endpoint for tags with stats

[acelaya]

Create a new endpoint to get tags with stats, let's say GET /tags/stats, which effectively deprecates GET /tags?withStats=true.

That endpoint would return a payload like this:

{
  "tags": {
    "data": [
      {
        "tag": "games",
        "shortUrlsCount": 10,
        "visitsCount": 521
      },
      {
        "tag": "shlink",
        "shortUrlsCount": 7,
        "visitsCount": 1087
      }
    ],
    "pagination": {...}
  }
}