Exclude Archived repos from Dashboard Milestones (#19882)

Milestones in archived repos should not be displayed on `/milestones`. Therefore
we should exclude these repositories from milestones page.

Fix #18257

Signed-off-by: Andrew Thornton <art27@cantab.net>

models/fixtures/repository.yml

@@ -618,7 +618,8 @@
   num_forks: 0
   num_issues: 1
   num_milestones: 1
-  is_mirror: false
+  is_mirror:
+  is_archived: false
 
 -
   id: 43