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
