validate will send forem feeds importer user agent, just like import (#16925)

follow on to #15942 which enabled downloading existing DEV feeds, this
allows setting a feed url to DEV.

It's possible this user agent should be brought inline with the
tags choice of community name and url, but in the short term I'm
making this consistent with the existing feed importer.

app/services/feeds/validate_url.rb

@@ -11,7 +11,9 @@ module Feeds
     def call
       return false if feed_url.blank?
 
-      xml = HTTParty.get(feed_url, timeout: 10).body
+      xml = HTTParty.get(feed_url,
+                         timeout: 10,
+                         headers: { "User-Agent" => "Forem Feeds Importer" }).body
       Feedjira.parse(xml)
 
       true