coalesce wrappers without intermediate text nodes

[jenseng]

This doesn't really affect functionality, but it does make for nicer translation strings... Consider:

<p translate="yes">Hello <b><i>{user}</i></b></p>

Currently that yields the translation string:

"Hello ** *%{user}* **"

The double wrappers add no value for translators, and are more likely to get screwed up. We should coalesce them so that the translators see:

"Hello *%{user}*"

i18nliner-handlebars and canvas_react_i18n both do this, we should too.

Even better would be no wrappers, if all that is contained is a placeholder, but we can save that for another ticket 😄