* Fix #2672 - Connect signed PuSH subscription requests to instance domain

Resolves #2739

* Fix return of locate_subscription

* Fix tests

* 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)

* Dont use raise_error by itself (avoids warning)

* Add coverage for AccountFilter

* Improve coverage and refactor for Subscription#lease_seconds

* Improve coverage and refactor for NotificationMailer

* Simplify assignment of min/max threshold on subscription

* Fix subscription expiration condition

* dry and add spec

* add annotate to Gemfile

* rails g annotate:install

* configure annotate_models

* add schema info to models

* fix rubocop to add frozen_string_literal

* Add `required: true` option to foreign column

* Fixes NoMethodError

```
> Favourite.new.valid?
NoMethodError: undefined method `reblog?' for nil:NilClass
```