Store both Markdown and rendered HTML. Closes #744.

Ok so here is the mighty solution to #744.

I normalized 2 of the database fields. Posts and pages now both have a `markdown` and `html` column. Instead of a mix of `content` and `html`, which was a little confusing.

I **kept the language files the same**, which means they still have the word `content` in their key names. I don't see this as all bad, since we don't need to change any language files at all if the key remains the same. It's up to the team to decide if it should change.

Everyone should see a nice bump in rendering time!