Fix feed formatting so it passes w3c validation

Feeds generated by Funkwhale do not pass validation with the [w3c
validator](https://validator.w3.org/feed/check.cgi?url=https%3A%2F%2Fpodcast.midline.pl%2Fapi%2Fv1%2Fchannels%2FMidline%2Frss).
This commit addresses the problems identified during validation:

1. The `isPermalink` is not recognized -> changed it to
   `isPermaLink` (capital "L")

2. `itunes:summary` and `itunes:subtitle` are 256 characters after
   truncating, but the maximum is 255. The truncating function trims the
   text to 255 chars, but then adds `…`, so the text is one character
   too long

3. The tags within `itunes:keywords` are now separated with commas
   instead of spaces (https://validator.w3.org/feed/docs/warning/InvalidKeywords.html)