Rename `source_url`

[eigenfoo]

...to something more descriptive! It's not always a URL - sometimes it can be a file path, such as with nytimes (e.g. https://cryptics.georgeho.org/data/clues/575156)

[eigenfoo]

The name should be source_location, in keeping with #26

[eigenfoo]

This would mean not just renaming all instances of source_url in the code, but also the column in cryptics.sqlite3. Should have a separate branch for this.