Improve performance of /with_tag.json endpoint

This changes the `/with_tag.json` endpoint to only return the details necessary for the browse pages (top-level details + description field).

Some crude tests on my dev VM produced the following:

`ab -n 10 -c 1 'http://contentapi.dev.gov.uk/with_tag.json?section=driving%2Fteaching-people-to-drive'`

Original code:
```
Connection Times (ms)
              min  mean[+/-sd] median   max
Connect:        0    0   0.0      0       0
Processing:  3431 3923 350.7   4065    4417
Waiting:     3430 3922 350.7   4064    4416
Total:       3431 3923 350.7   4065    4417
```

New code:
```
Connection Times (ms)
              min  mean[+/-sd] median   max
Connect:        0    0   0.0      0       0
Processing:   157  177  14.5    178     195
Waiting:      157  177  14.5    178     195
Total:        157  177  14.5    178     195
```

Digging revealed that there were 2 things that were causing the slowdown:

1. Loading in all the related artefacts and tags
2. Processing the Govspeak fields

These added around 2s each.