* Don't create podcast episodes without a media_url
There is an early return from add_media_url to prevent using an empty
enclosure_url, which avoids errors in
`GetMediaUrl.call(a_blank_value)`, however there's also a not null
constraint on podcast_episodes.media_url in the database, which causes
upsert to raise an error.
Returning early here avoids the error (which had halted the worker).
See also
https://app.honeybadger.io/projects/66984/faults/80833402
This error happens regularly, 96 times per day in DEV, it looks like at least
one podcast feed_url is for an rss feed filtered by /tag/podcast/,
(and at least one post was tagged "podcast" but didn't include any
media to get).
My understanding is at worst we'll ignore the error silently.
* Add test cases
given a blog post (not a podcast episode, no media url), don't raise
any error, don't create any episode.
Included codepunk.io feed that was showing this issue initially.
* Use sluggerize to produce a slug from podcast episode title
This is essentially the process we use for Article#title_to_slug
without the random trailing bits on the end (since podcast episodes
had not worked that way previously).
* Add test case for slug generation
* Oh, the test only works if you checkin the fixture, too
