Add wikis

[hatkirby]

Wikis are entries that should be similar to blogs, mainly differing in semantics & presentation. One notable difference is that they should have the option to display a table of contents above the first subheading (similar to what MediaWiki does). Another option could be to show a revision history if we decide to keep revisions, but I'm not sure about that yet. Currently unsure as to how exactly the presentation of wikis should differ from blogs.