Add ability to filter instance by those that are currently available

app/models/instance_filter.rb

@@ -71,6 +71,8 @@ class InstanceFilter
       Instance.where(domain: DeliveryFailureTracker.warning_domains)
     when 'unavailable'
       Instance.joins(:unavailable_domain)
+    when 'available'
+      Instance.left_outer_joins(:unavailable_domain).where(unavailable_domain: { domain: nil })
     else
       raise Mastodon::InvalidParameterError, "Unknown availability: #{value}"
     end

app/views/admin/instances/index.html.haml

@@ -27,6 +27,7 @@
     %strong= t('admin.instances.availability.title')
     %ul
       %li= filter_link_to t('admin.instances.delivery.all'), availability: nil
+      %li= filter_link_to t('admin.instances.delivery.available'), availability: 'available'
       %li= filter_link_to t('admin.instances.delivery.failing'), availability: 'failing'
       %li= filter_link_to t('admin.instances.delivery.unavailable'), availability: 'unavailable'
 

config/locales/en.yml

@@ -521,6 +521,7 @@ en:
         instance_statuses_measure: stored posts
       delivery:
         all: All
+        available: Available
         clear: Clear delivery errors
         failing: Failing
         restart: Restart delivery