Allow customization of assignment email subjects and content

### Problem Statement

Currently, Sentry supports project-wide email subject prefixes via mail:subject_prefix and customizable subject templates for issue alert emails via mail:subject_template. However, assignment activity notifications use a fixed subject format that cannot be customized.

This limitation prevents teams from distinguishing assignment notifications by environment or other context (e.g., staging vs. production) at a glance.

### Solution Brainstorm

Extend the existing mail:subject_template functionality (or introduce a similar option) to support assignment emails.

### Product Area

Alerts

Start by tracing the existing mail:subject_prefix and mail:subject_template handling, then locate the assignment activity notification path. Define how the existing template functionality or a similar option should apply to assignment emails, and verify that assignment subjects and content can be customized.

Written by the indexing model from the issue text.