Handle FTP [more] gracefully

We currently have a few FTP directories we monitor, but we don’t actually handle them very well.

The real issues under the hood:

When we get FTP listings out of Versionista, we wind up storing them as application/octet-stream, which means we wind up treating them like binary data later on. We could store them as text/plain (see Wayback below) or we could make something more specific. (See also edgi-govdata-archiving/web-monitoring-versionista-scraper#166)
When we get FTP listings out of Wayback, we wind up storing them as text/plain, which at least makes them displayable and diffable, but we don’t diff them in a particularly useful way:
In the UI, we are parsing mime types poorly and we read wayback’s text/plain as text/html, so we don’t give it the most friendly visualization (edgi-govdata-archiving/web-monitoring-ui#322).
We could diff these as plain text, which is moderately useful:

But it might be nice to have a fancier diff in this case, like we do for links. I think we’d probably need a new mime-type for this (e.g. text/x-wm-ftp-directory or text/ftp-dir-listing [this is what Versionista appears to be sending, which is non-standard but also used by some other tools]), though we could possibly also detect that it’s an FTP listing by checking version.capture_url.startsWith('ftp://') in the UI.

edgi-govdata-archiving / web-monitoring