more specific annotation of json

We annotate json as we serve it so that the 
client can adjust rendering. Example: green 
border for pages served from plugins.

This commit moves the logic for applying the
plugin annotation to a point where we know 
for sure that we are looking for plugin pages.

This fixes one issue mentioned in  https://github.com/fedwiki/wiki-plugin-calculator/pull/2

This pull is pending testing. @nrn has promised to help me straighten out my server build when he returns from jsconf tonight.