HTTP signatures (#4146)

* Add Request class with HTTP signature generator

Spec: https://tools.ietf.org/html/draft-cavage-http-signatures-06

* Add HTTP signature verification concern

* Add test for SignatureVerification concern

* Add basic test for Request class

* Make PuSH subscribe/unsubscribe requests use new Request class

Accidentally fix lease_seconds not being set and sent properly, and
change the new minimum subscription duration to 1 day

* Make all PuSH workers use new Request class

* Make Salmon sender use new Request class

* Make FetchLinkService use new Request class

* Make FetchAtomService use the new Request class

* Make Remotable use the new Request class

* Make ResolveRemoteAccountService use the new Request class

* Add more tests

* Allow +-30 seconds window for signed request to remain valid

* Disable time window validation for signed requests, restore 7 days
as PuSH subscription duration (which was previous default due to a bug)

app/workers/pubsubhubbub/delivery_worker.rb

@@ -33,9 +33,9 @@ class Pubsubhubbub::DeliveryWorker
   end
 
   def callback_post_payload
-    HTTP.timeout(:per_operation, write: 50, connect: 20, read: 50)
-        .headers(headers)
-        .post(subscription.callback_url, body: payload)
+    request = Request.new(:post, subscription.callback_url, body: payload)
+    request.add_headers(headers)
+    request.perform
   end
 
   def blocked_domain?
@@ -48,13 +48,12 @@ class Pubsubhubbub::DeliveryWorker
 
   def headers
     {
-      'User-Agent' => 'Mastodon/PubSubHubbub',
       'Content-Type' => 'application/atom+xml',
-      'Link' => link_headers,
+      'Link' => link_header,
     }.merge(signature_headers.to_h)
   end
 
-  def link_headers
+  def link_header
     LinkHeader.new([hub_link_header, self_link_header]).to_s
   end