Improve community-wiki functionality

[slel]

Three things Askbot is missing with respect to the "community wiki" functionality, in my opinion:

• a good description of when to check this box, that a user would see when posting
  a question or answer; including the recommendation to not check the box if in doubt;
• a way for site administrators to modify the "community wiki" status of a question
  after it has been posted (i.e., turn this on or off for a question when the original
  poster made the wrong choice);
• a way to search an askbot-powered site for questions tagged as "community wiki"
  (one can search other tags, but not that special one, with the site's search box); see
  http://askbot.org/en/question/12587/view-all-community-wiki-questions-on-an-askbot-site/;

The present feature request was initially written as an answer to a question on Ask Askbot:
https://askbot.org/en/question/6643/what-is-the-intention-of-community-wiki-questions/?answer=15451#post-id-15451

Separate issues could be opened for each specific feature, and progress tracked here.