FEATURE: add support for tag group search

The behaviour of #TERM in search has been amended

1. We try category or subcategory slugs
2. We try tags
3. We try tag-groups

The term `hello #my-group` will search for all posts tagged with any of
the tags in the tag group `My Group`

Future work may be introducing a slug cache here or caching it in the table
but the assumption is that the number of tag groups will not be huge

app/models/tag_group.rb

@@ -36,11 +36,18 @@ class TagGroup < ActiveRecord::Base
     @permissions = TagGroup.resolve_permissions(permissions)
   end
 
-  def self.resolve_permissions(permissions)
-    everyone_group_id = Group::AUTO_GROUPS[:everyone]
-    full = TagGroupPermission.permission_types[:full]
+  # TODO: long term we can cache this if TONs of tag groups exist
+  def self.find_id_by_slug(slug)
+    self.pluck(:id, :name).each do |id, name|
+      if Slug.for(name) == slug
+        return id
+      end
+    end
+    nil
+  end
 
-    mapped = permissions.map do |group, permission|
+  def self.resolve_permissions(permissions)
+    permissions.map do |group, permission|
       group_id = Group.group_id_from_param(group)
       permission = TagGroupPermission.permission_types[permission] unless permission.is_a?(Integer)
       [group_id, permission]