Normalize lone CRs as new lines

[josefarias]

One of our users ran into a scenario where copying content from Skype into a Trix editor in Chrome would record line breaks as a lone \r character. This results in the line breaks being omitted because we're not normalizing \r into a new line.

We already normalize \r\n as a \n character. This change makes it so we also normalize \r into \n.

Here's the contents of my clipboard after trying copying from Skype myself:

{
  "text/plain": "José, 02:19 PM\rTest message #1\rContent in a new line\r\rJosé, 02:19 PM\rTest message #2\rContent in another new line"
}

https://3.basecamp.com/2914079/buckets/27/card_tables/cards/6628261881

[kevinmcconnell]

This looks good to me 👍 The new behaviour seems consistent with the other editors I tried.