Return leading image for {{content words="0"}}

Closes #3884
- This PR includes a reimplementation of Downsize@0.0.5 with `words: "0"`. It uses the same parsing methods, therefore guarunteeing identical behavior.
- Tests have been updated to be more extensive.
Merge pull request #3888 from felixrieseberg/iss3884-content

Return leading image for {{content words="0"}}
Add checks for packages and contentPath to startup

Refs #3864
- Make sure that require() is able to resolve all dependencies
  listed in package.json.  If packages are missing halt the
  bootstrap process and display an error and help message for user.
- Check that contentPath and its subdirectories exist with the correct
  permissions.
- Check sqlite3 database file is set for read/write access.