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