Add mailers for org admin alerts

[SamJamCul]

What problem does this pull request solve?

Trello card: https://trello.com/c/FLOb2XJa/2849-add-mailers-for-org-admin-alerts

Adds the Notify templates (which you can find here, and creates mailers that map to them.

Some of the templates are subject to change based on content input, so it might need a bit of tweaking before it goes live.

They are hidden behind the feature flag, because they're not currently in use anywhere. There may be some opportunities to reduce duplication in some of the mailers, but that will be contingent on the pending modifications to the templates.

Things to consider when reviewing

• Ensure that you consider the wider context.
• Does it work when run on your machine?
• Is it clear what the code is doing?
• Do the commit messages explain why the changes were made?
• Are there all the unit tests needed?
• Do the end to end tests need updating before these changes will pass?
• Has all relevant documentation been updated?

[stephencdaly]

I've left some super nitpicky comments about the naming of some methods just because I found them a little confusing to read. Feel free to disagree with them though.

Other than that looks good. I haven't checked all the content in the emails, but I've checked the template IDs match up.

[sonarqubecloud]

Quality Gate passed

Issues
6 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
12.2% Duplication on New Code

See analysis details on SonarQube Cloud

[github-actions]

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