wiki topic

See this topic for details: https://meta.discourse.org/t/wiki-topic-mini-spec/13966

Functionality should work more or less as in requirements in above post.

Layout is my invention - it will most probably require some polishing.

There are also 2 new translations added: `post.controls.admin` and `post.wiki`. I don't know how the process of adding translations looks like (should they be added directly to all language files, to english file only or via external translation system) so for now I've added them only in the code (translation files need to be updated with these 2 new keys).