Don’t let _set_status_filters mutate original list

Mutating stuff is scary and prone to problems. Better for it to
return a new list instead.

app/templates/views/notifications.html

@@ -14,9 +14,9 @@
   <h1 class="heading-large">
 
     <span class="visually-hidden">
-      {%- if request_args.get('status') != 'delivered,failed' -%}
+      {%- if status != 'delivered,failed' -%}
         {%- for label, option, _, _ in status_filters -%}
-          {%- if request_args.get('status', 'delivered,failed') == option -%}{{label}} {% endif -%}
+          {%- if status == option -%}{{label}} {% endif -%}
         {%- endfor -%}
       {%- endif -%}
     </span>
@@ -29,7 +29,7 @@
     {{ pill(
       'Status',
       status_filters,
-      request_args.get('status', '')
+      status
     ) }}
   </div>