views: add embed_wiki helper.

Also remove options to configure names of wiki notice pages. These names
generally don't need to be changed and we already hardcode links to wiki
pages in other places anyway.

app/helpers/application_helper.rb

@@ -163,6 +163,12 @@ module ApplicationHelper
     html.html_safe
   end
 
+  def embed_wiki(title, **options)
+    wiki = WikiPage.find_by(title: title)
+    text = format_text(wiki&.body)
+    tag.div(text, class: "prose", **options)
+  end
+
   def dtext_field(object, name, options = {})
     options[:name] ||= name.capitalize
     options[:input_id] ||= "#{object}_#{name}"