Delete tag uses the wrong ID

The current logic for the tags uses the image ID for tag instead of the tag ID.

```
[06/Nov/2018:01:54:32 +0000] "DELETE /tags/pHR1ajkzR/ HTTP/1.0" 404 89 "-" "TaggingTracker/1 CFNetwork/975.0.3 Darwin/17.7.0"
```

```
[2018-11-06 01:54:32 +0000] [53] [DEBUG] DELETE /tags/pHR1ajkzR/
```

We should switch to using the tag ID instead.
