The feed does not pass w3c validation

/label Type: Bug Status: Need triage

Steps to reproduce

Describe the steps to reproduce the issue, like:

1. Get the URL of the podcast feed, like so: <domain>/api/v1/channels/<channel name>/rss
2. Enter it into the W3C feed validator

What happens?

Validation errors appear (attached below), and that might make the feed incompatible with certain podcast services.

What is expected?

The feed passes validation

Context

Funkwhale version(s) affected: 1.0

Validator errors:

This feed does not validate.

    line 2, column 1093: length of 256 exceeds the maximum allowable for itunes:subtitle of 255 [help]

        ...  zrozumienie problemów, które tworzą. J…</itunes:subtitle><itunes:summar ...
                                                     ^

    line 2, column 3087: Unexpected isPermalink attribute on guid element (2 occurrences) [help]

        ...  oraz chińskich zegarkach</itunes:title><guid isPermalink="false"><![CDA ...
                                                     ^

    line 2, column 3161: guid must be a full URL, unless isPermaLink attribute is false: 443a24c8-d253-4547-8346-bca25357e6d5 (2 occurrences) [help]

        ... [443a24c8-d253-4547-8346-bca25357e6d5]]></guid><pubDate>Thu, 08 Oct 2020 ...
                                                     ^

    line 2, column 3876: length of 256 exceeds the maximum allowable for itunes:subtitle of 255 (2 occurrences) [help]

        ... urencyjny, że kierowcy pracujący jako d…</itunes:subtitle><itunes:summar ...
                                                     ^

In addition, interoperability with the widest range of feed readers could be improved by implementing the following recommendations.

    line 2, column 1953: itunes:summary should not contain HTML: strong [help]

        ... m/show/2BSREI0uQcADS8W0RHrkxB</a></p>]]></itunes:summary><description>"I ...
                                                     ^

    line 2, column 2867: Use commas to separate keywords [help]

        ... ść security technologia technology users</itunes:keywords><item><title># ...
                                                     ^

    line 2, column 4496: itunes:summary should not contain HTML: p (2 occurrences) [help]

        ... //www.midline.pl/prasowka-10/</a></p>]]></itunes:summary><description>W  ...
                                                     ^