feat: more mail sending backends and plugability (#9608)

* [FR] Improve Email handeling
Fixes #7950

* extend implementation of email thread and message models

* add missing args

* add unit test

* increase test coverage

* make key not necessary

* do not consider in coverage

* add email apis

* Add email admin

* fix email configuration check

* improve rendering

* squash migrations

* add config value overview

* log if mails were send

* add additional headers

* fix api unit test

* fix url resolving

* add InvenTree specific task to issue mails
required to extend sending options (prio, reply to)

* use internal sending task to keep telemetry cleaner

* add prio handling

* add plugin handling

* add setting

* factor plugin method out

* add typing

* move function

* bump version

* fix import path

* add a test for the test endpoint

* fix checking logic

* Add anymail sending / tracking handling

* add more ordering fields to api

* remove unneeded assingment

* add basic docs

* handle incoming emails with anymail

* Add inbox handling
Closes https://github.com/inventree/InvenTree/issues/7951

* add list of supported ESPs

* add better error transparency when sending fails

* add missing migration

* combine migrations back down

* fix todos

* fix qc export

* fix missing model props

* add tests

* ensure things are passed as a list

* fix list formatting

* fix deps

* move tests

* add testing with anymail

* allow handling of priority and headers

* add test for events

* add test for inbound messages

* rename variable

* increase coverage

* fix format

* add setting doc

* fix link

* rename fnc

* disable pro test

* make messages clearer

* fix doc syntax

* fix assign

* fix test

* revert test disablement

* add enum

* disable check for now

* try changing test around

* add incoming mail processing

* fix import

* add docs

* Fix mail.md

* bump deps

* fix api version

src/frontend/lib/enums/ApiEndpoints.tsx

@@ -235,5 +235,8 @@ export enum ApiEndpoints {
   error_report_list = 'error-report/',
   project_code_list = 'project-code/',
   custom_unit_list = 'units/',
-  notes_image_upload = 'notes-image-upload/'
+  notes_image_upload = 'notes-image-upload/',
+  email_list = 'admin/email/',
+  email_test = 'admin/email/test/',
+  config_list = 'admin/config/'
 }