This commit renames "Message templates" to "Notification templates"
in the user interface as it suggests that these templates cannot
be used to template anything other than the message. However, message
templates are much more general and can be used to template other fields
too such as the subject of an email, or the title of a Slack message.
* Test composition simplification from last PR
* Policies use proper API model everywhere
* Expose policy provenance in API, miss some dep injection
* Complete injection
* fix args
* Tests for provenance value
* Extract test helpers so tests are very readable
* Single source adapter struct that was copied in 3 places
* Drop redundant test
* Resolve merge conflicts on changelog
* Refactor GET am config to be extensible
* Extract post config route
* Fix tests
* Remove temporary duplication
* Fix broken test due to layer shift
* Fix duplicated error message
* Properly return 400 on config rejection
* Revert weird half method extraction
* Move things to notifier package and avoid redundant interface
* Simplify documentation
* Split encryption service and depend on minimal abstractions
* Properly initialize things all the way up to the composition root
* Encryption -> Crypto
* Address misc feedback
* Missing docstring
* Few more simple polish improvements
* Unify on MultiOrgAlertmanager. Discover bug in existing test
* Fix rebase conflicts
* Misc feedback, renames, docs
* Access crypto hanging off MultiOrgAlertmanager rather than having a separate API to initialize
