updating help text and labels

[brandonhundt]

Closes #934

• [x] Feed owner help text: Probably best to avoid the term iTunes
• [x] A lot of fields have help text that provides no additional information. For example, the Managing Editor Name says "set the managing editor name for this podcast". I don't think that helps a user who has already seen the input field and knows it's called Managing Editor Name. May be better to remove the (?) buttons when there's no significant help text available.
• [x] Title help text is the only one on the whole page I've found that is in the form of a question
• [x] Category help text, should probably call them "Apple Podcast categories" and link to https://podcasters.apple.com/support/1691-apple-podcasts-categories
• [x] Description help text says you can add images, but that doesn't seem to be true
• [x] Production notes, if this is just internal and never visible to listeners, I would explicitly say that in the help text
• [x] Explicit help text, replace iTunes with Apple Podcast
• [x] Inconsistency around using the terms Link and URL

I'm open to more suggestions. As things come up.