Expand email customization and bring to namespace level

Summary

Customers often need to add detail to email notifications to standardize workflow, meet internal standards, or meet regulatory requirements.

These customizations can take 2 primary forms:

1. Customizing the generic email wrapper
2. Customizing the content of the email notification

This issue focuses only on the generic email wrapper (1), solutions for 2 would be complimentary to any solutions here.

Depending on the use case, customization may be required for all emails (instance level) or just for emails resulting from certain groups or projects.

Proposal

Long-term vision: Custom templates (validation needed to support value)

Near-term vision: Allow customizing the subject and body wrapper at both the instance and group/project levels, with option to cascade settings.

Instance	Group/Project	MVC (Instance)

MVC1:

• Add "Subject starter" to instance level (Admin > Preferences > Email)
• (Nice to have) Utilize color selection in System header and footer when set to appear in emails (utilize same background/text color as the system banners if multipart is enabled)