Resolve "The feed does not pass w3c validation"

Feeds generated by Funkwhale do not pass validation with the w3c validator. 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)

Closes #1250 (closed)