Help text misleading for translators

The help text for the form element allowing to override a block's title is the following:

Override the default title for the block. Use <none> to display no title, or leave blank to use the default block title.

The whole thing is passed through t(), so some translators have translated all of it, including <none>. Of course, if you use <keine> (German for <none>, which is actually suggested by the German translation of the help text) you don't get the intended effect. The placeholder <none> is just not meant to be translated. This patch simply wraps it in a t() placeholder so that it doesn't get translated.