Add featured tags and return featured books in extras API

MANUAL-UPGRADE:

1. Insert featured tags: (tag ids may need to be adjusted)

```
INSERT INTO tags VALUES (7, 'OpenStax Featured', 'featured');
INSERT INTO tags VALUES (8, 'CNX Featured', 'featured');
```

2. Link books to the featured tags by inserting data into the moduletags
table.

3. Upload the cover image for featured books to the module_files and
files tables with filename "featured-cover.png"
Merge pull request #266 from Connexions/featured-links-2

Add featured tags and return featured books in extras API