Change User-Agent of link preview fetching service to include "Bot" (#14248)

This forces Twitter to render OpenGraph tags in the response

app/services/fetch_link_card_service.rb

@@ -45,7 +45,7 @@ class FetchLinkCardService < BaseService
   def html
     return @html if defined?(@html)
 
-    Request.new(:get, @url).add_headers('Accept' => 'text/html').perform do |res|
+    Request.new(:get, @url).add_headers('Accept' => 'text/html', 'User-Agent' => Mastodon::Version.user_agent + ' Bot').perform do |res|
       if res.code == 200 && res.mime_type == 'text/html'
         @html = res.body_with_limit
         @html_charset = res.charset