Handle unknown categories from global search

Signed-off-by: Thomas Citharel <tcit@tcit.fr>

lib/mobilizon/events/categories.ex

@@ -4,11 +4,27 @@ defmodule Mobilizon.Events.Categories do
   """
   import Mobilizon.Web.Gettext
 
+  @default "MEETING"
+
+  @spec default :: String.t()
+  def default do
+    @default
+  end
+
   @spec list :: [%{id: atom(), label: String.t()}]
   def list do
     build_in_categories() ++ extra_categories()
   end
 
+  @spec get_category(String.t() | nil) :: String.t()
+  def get_category(category) do
+    if category in Enum.map(list(), &String.upcase(to_string(&1.id))) do
+      category
+    else
+      default()
+    end
+  end
+
   defp build_in_categories do
     [
       %{