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)

spec/workers/pubsubhubbub/delivery_worker_spec.rb

@@ -59,7 +59,7 @@ describe Pubsubhubbub::DeliveryWorker do
         'Content-Type' => 'application/atom+xml',
         'Host' => 'example.com',
         'Link' => "<https://#{Rails.configuration.x.local_domain}/api/push>; rel=\"hub\", <https://#{Rails.configuration.x.local_domain}/users/#{subscription.account.username}.atom>; rel=\"self\"",
-        'User-Agent' => 'Mastodon/PubSubHubbub',
+        'User-Agent' => 'http.rb/2.2.2 (Mastodon/1.4.7; +https://cb6e6126.ngrok.io/)',
       }.tap do |basic|
         known_digest = OpenSSL::HMAC.hexdigest(OpenSSL::Digest.new('sha1'), subscription.secret.to_s, payload)
         basic.merge('X-Hub-Signature' => "sha1=#{known_digest}") if subscription.secret?