HTML renderer supports HTML entities while other renderers don't

[andrewleader]

We have a consistency issue in regards to the handling of HTML entities ( , <, >, etc).

Repro payload

{
    "type": "AdaptiveCard",
    "body": [
        {
            "type": "TextBlock",
            "text": "< &lt;"
        }
    ],
    "$schema": "http://adaptivecards.io/schemas/adaptive-card.json",
    "version": "1.0"
}

HTML renderer

In the HTML JavaScript renderer, as seen from the Visualizer, these are understood and displayed.

Other renderers

In the UWP (and I assume other) renderers, these are NOT understood and displayed as raw text.

We should have consistent behavior here. Looking at CommonMark's specification of Markdown, HTML entities should be understood and processed by Markdown. Considering we are supporting markdown in our text blocks, we should be doing this?

[andrewleader]

Duping to #1984