Adds domain support to OSEM

- This is a very basic support. All it affects is the listings
  on the homepage. Every conference can set a domain, and as
  long as the request host matches this domain, the conference
  will be shown on the list
- If a conference does not have a domain set (which will be the
  case for older conferences), such conferences are shown
  always
- Conference creation and edits now have an optional domain field
- Also adds corresponding migrations (+index)

app/views/admin/conferences/edit.html.haml

@@ -9,6 +9,7 @@
     = semantic_form_for(@conference, :url => admin_conference_path(@conference.short_title),:html => {:multipart => true}) do |f|
       = f.input :title, :hint => "The full title of the conference, e.g. 'openSUSE Conference 2014'"
       = f.input :short_title, :hint => "A short title, e.g. 'oSC14', to be used in URLs"
+      = f.input :domain, hint: "Optional domain on which the conference will be visible. This only affects the conference listings."
       = f.input :description, hint: markdown_hint('A description of the conference.'), input_html: { rows: 5, data: { provide: 'markdown-editable' } }
       = f.input :color, :hint => "The color will be used eg for the dashboard.", :input_html => {:size => 6, :type => "color"}
       = f.label 'Conference Logo'