Localize notification_utils to the API

This changeset pulls in all of the notification_utils code directly into the API and removes it as an external dependency.  We are doing this to cut down on operational maintenance of the project and will begin removing parts of it no longer needed for the API.

Signed-off-by: Carlo Costino <carlo.costino@gsa.gov>

README.md

@@ -431,23 +431,6 @@ In either situation, once you are finished and have verified the dependency
 changes are working, please be sure to commit both the `pyproject.toml` and
 `poetry.lock` files.
 
-### Keeping the notification-utils Dependency Up-to-Date
-
-The `notifications-utils` dependency references the other repository we have at
-https://github.com/GSA/notifications-utils - this dependency requires a bit of
-extra legwork to ensure it stays up-to-date.
-
-Whenever a PR is merged in the `notifications-utils` repository, we need to make
-sure the changes are pulled in here and committed to this repository as well.
-You can do this by going through these steps:
-
-- Make sure your local `main` branch is up-to-date
-- Create a new branch to work in
-- Run `make update-utils`
-- Commit the updated `poetry.lock` file and push the changes
-- Make a new PR with the change
-- Have the PR get reviewed and merged
-
 ## Known Installation Issues
 
 ### Python Installation Errors