Read links as attachments to .url

[gwiedeman]

The problem the component solves

In some cases, emails can come with URL links as attachments. We should optionally save these as .url files in the attachment subdirectory.

Relevant part of mailbag spec?

4.3.2 Links as attachments

Type of component

• [ ] Core
• [ ] Input
• [x] Attachments
• [ ] Derivatives conversion
• [ ] Reporting/Exporting
• [ ] GUI
• [ ] Distribution

Expected contribution

• [X] Pull Request
• [ ] Comment with proposed solution

Major challenges or things to keep in mind

Poses some big security issues we'll have to think about.