Fix that the [text-direction=rtl] tag does not work below Markdown lists

[tanius]

Currently, the [text-direction=rtl] … [/text-direction] tag will not have any effect on the "cooked HTML" output of Discourse (so, no right-aligning) if it follows directly after a Markdown list, like this:

* Markdown list item 1
* Markdown list item 2
[text-direction=rtl]
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[/text-direction]

It will however work when separating it from the Markdown list with an empty line, like this:

* Markdown list item 1
* Markdown list item 2

[text-direction=rtl]
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[/text-direction]

This should not be required, as whitespace-significant markup or programming languages are confusing and not intuitive. Markdown itself is whitespace significant (newline for a paragraph etc.), but any tags should take higher priority.

If necessary, an implementation could be to pre-process the text and add the "missing" empty lines where needed.