Prevent Navigation Item from caching old link targets

[jmurty]

The NavigationItemPlugin and AccountsNavigationItemPlugin content plugins currently cache their output.

This is a problem because the rendered output will not be updated for some situations, for example if you change the URL location of a Page referenced by a NavigationItem the old URL will continue being rendered until you clear the cache.

The brute-force fix is to add the cache_output = False attribute to these plugins.

A better fix, and one we should start using in general, is to figure out how to generate a sensible cache key and override the get_output_cache_key() method of ContentPlugin so that we can benefit from caching but have that cache invalidated when appropriate.