User config shall take precedence over org config

[rafatower]

This is a pretty common use case for services:

• an org (such as team) has a default provider configured, but
• an individual user wants to override the org-level config with their own provider

cc/ @ethervoid

[rafatower]

Watch out, for quotas it'll work as 2 different checks. In pseudo-code

# Imagine user john_smith of the acme_org
acme_org.users.service_usage.reduce(:+1) < acme_org.max_quota_allowance or raise 'org has no quota left'
# AND...
john_smith.service_usage < john_smit.max_quota_allowance or raise 'user has exceeded their quota'

[rafatower]

Related: #280