make `BlockQuote` a SPAN instead of extending `MarkdownText` so that it can be a child of `MarkdownText`

[andycmaj]

we currently have:

<SectionBlock>
        <MarkdownText>{triggeringActivity.data.comment}</MarkdownText>
</SectionBlock>

// OR


<SectionBlock>
        <BlockQuote>{commentBody}</BlockQuote>
</SectionBlock>

we should be able to put BlockQuotes INTO markdown blocks.

<SectionBlock>
        <MarkdownText>
			Here's a comment from Dave: 

			<BlockQuote>{commentBody}</BlockQuote>
		</MarkdownText>
</SectionBlock>

hmmm can a BlockQuote be a CHILD of MarkdownText? practically, i thikn you'd sometimes want to be able to have markdown sections that CONTAIN block quotes and other things as well...

Originally posted by @andycmaj in https://github.com/asynchronous-dev/botany/pull/973