T105242: U is for urlencode

T105242: U is for urlencode

 * Urlencode and extension tags don't play nice. The current behaviour
   is pretty broken. For example,

      {{urlencode:<pre id="one">two</pre>}}

   outputs

       <pre id="one">two</pre>

   which isn't exactly urlencoded.

 * Just drop the unexpanded content; anything more seems unsafe. There's
   precedent for this already in anchorencode.

 * Noticed in the infobox caption for enwiki/%22F%22_Is_for_Fugitive

Change-Id: I0ec5d2fdeb71f762036464f65029afd029e8b4a1