feat: Better Mail Methods

[matmair]

This introduces models to track mail delivery and receiving - and mail processing.

Todo:

• Mail receiving
• Passing received mails to plugins
• Passing outgoing mails to plugins
• Admin UI to view mails
• doc improvements
• maybe: send test mail

Closes #7950
Closes #7951

I have validated sending against:

• SMTP with cpanel
• SMTP with Postfix
• Sending / receiving with Mailgun
• NOT - avoid them! - Sending / receiving with Sendgrid
• Sending with Mailjet

Waiting for #9596 to add proper grouping for the new email pane

[netlify]

✅ Deploy Preview for inventree-web-pui-preview canceled.

Name	Link
🔨 Latest commit	8944883
🔍 Latest deploy log	https://app.netlify.com/projects/inventree-web-pui-preview/deploys/682e5eed7d9e5d0008835743

[codecov]

Codecov Report

Attention: Patch coverage is 97.70408% with 9 lines in your changes missing coverage. Please review.

Project coverage is 86.38%. Comparing base (3d71be3) to head (8944883).
Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
src/backend/InvenTree/InvenTree/helpers_email.py	40.00%	6 Missing ⚠️
src/backend/InvenTree/InvenTree/middleware.py	50.00%	2 Missing ⚠️
src/backend/InvenTree/InvenTree/settings.py	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9608      +/-   ##
==========================================
+ Coverage   86.35%   86.38%   +0.03%     
==========================================
  Files        1229     1235       +6     
  Lines       53966    54386     +420     
  Branches     2259     2259              
==========================================
+ Hits        46601    46980     +379     
- Misses       6793     6834      +41     
  Partials      572      572              

Flag	Coverage Δ
backend	88.42% <97.70%> (+0.10%)	⬆️

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[matmair]

@SchrodingersGat should i split out the admin center docs into a separate PR?

[SchrodingersGat]

Probably makes for easier review

[matmair]

Probably makes for easier review

#9646