Adds enum certificate_source to pages_domains table
with default manually_uploaded

Mark certificates as 'gitlab_provided'
if the were obtained through Let's Encrypt

Mark certificates as 'user_provided' if they were uploaded through
controller or api

Only show private key in domain edit form if it is 'user_provided'

Only show LetsEncrypt option if is enabled by application settings
(and feature flag)

Refactor and fix some specs to match new logic

Don't show Let's Encrypt certificates as well

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