Allow dictionary with license URL

[migurski]

First task for openaddresses/openaddresses-ops#7: allow both strings and dictionaries for source license data. Data is single field for now (“A URL or string describing the license for the dataset”), so support only URL and text tags alongside old-style single strings:

"license": <URL or string>

Naively-converted source:

"license": {
    "text": <URL or string>
}

Source with URL detected:

"license": {
    "url": <URL>
}

Source with additional details:

"license": {
    "url": <URL>,
    "text": <string>
}