feat: Add Mandrill Subaccount support

[amirtds]

Change description

Adding Mandrill Subaccount support. We tried to use dedicated IP for PSU but Mandrill team told us the best way to handle large number of emails is subaccount. Back in 2021 I added the subaccount support to Hawthorn but seems it never made it to Juniper. #1035

Type of change

• Bug fix (fixes an issue)
• New feature (adds functionality)

Checklists

Development

• Lint rules pass locally
• Application changes have been tested thoroughly
• Automated tests covering modified code pass

Security

• Security impact of change has been considered
• Code follows company security practices and guidelines

Code review

• Pull request has a descriptive title and context useful to a reviewer. Screenshots or screencasts are attached as necessary
• "Ready for review" label attached and reviewers assigned
• Changes have been reviewed by at least one other contributor
• Pull request linked to task tracker where applicable

[github-actions]

Checking git merge conflicts against https://github.com/edx/edx-platform.git

Comparing with	open-release/nutmeg.master
Benchmark conflicts with main	299
Current conflicts	299
Summary	Good work! No added conflicts.

Comparing with	master
Benchmark conflicts with main	318
Current conflicts	318
Summary	Good work! No added conflicts.

[bryanlandia]

lgtm