As proposed in #3378
* Experimental support for comments on the entire wiki (enable in the plugin config panel). Implemented as comments on $:/SiteTitle, with the comments appearing at the top of the story river
* Refactor the "add comment" button actions so that they can be reused
* Refactor the comment toolbar to be extensible via a system tag
* Add an $:/AdvancedSearch canned filter for accessing all comments
