content.body property replaces content.htmlString and content.html()

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Previously Content instances stored their body html property at `.htmlString` and also exposed an accessor at `.html()`. Mustache templates also then depended on the `{{{ htmlString }}}`.

Now, this property is stored at `content.body`, and templates have been updated accordingly. The `.html()` method was also removed.

Some tests were changed to accomodate this. I added some to make sure this works for `LivefyreContent` objects in the future.