* 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)

* Use I18n.locale instead of ":en"
* Reset I18n.locale value after locale changing tests

* Improve default language decision

This change allows to takes account of accepted language determined by
the user agent even if the custom default locale of the instance is
configured.

* Cover Localized more

* Fix code style

* Coverage for rate limit headers

* Move rate limit headers methods to concern

* Move throttle check to condition on before_action

* Move match_data variable into method

* Move utc timestamp to separate method

* Move header setting into smaller methods

* specs cleanup

* Remove methods from ObfuscateFilename

* Spec ObfuscateFilename