Create an .html file for all existing .msg files

**Jira Ticket**: ALCS-2202

As an ALCS user, I want to easily open emails in the browser and ensure that emails are saved in a file type that is not proprietary.

**Acceptance Criteria**:

- Run a migration to create an .html file from the content of existing .msg files
- Complete this for all .msg files in ALCS
- Keep the .msg files after the conversion

No files, tests, or entry points are named. Start by locating the migration or processing entry point for existing .msg files and inspect how email content is stored and read. Done means every existing .msg file has a corresponding browser-openable .html file while the original .msg files remain.

Written by the indexing model from the issue text.