Extract acme double to helper

Create ACME challanges for pages domains

* Create order & challange through API
* save them to database
* request challenge validation

We're saving order and challenge as one entity,
that wouldn't be correct if we would order certificates for
several domains simultaneously, but we always order certificate
per domain

Add controller for processing acme challenges redirected from pages

Don't save acme challenge url - we don't use it

Validate acme challenge attributes

Encrypt private_key in acme orders

Part of adding Let's Encrypt certificates for pages domains

Add acme-client gem

Client is being initialized by private key stored in secrets.yml
Let's Encrypt account is being created lazily.
If it's already created, Acme::Client just gets account_kid by
calling new_account method

Make Let's Encrypt client an instance
Wrap order and challenge classes