Fix composing the support letter with a malformed recipient

[Radiokot]

Purpose

Fix composing a letter with a malformed recipient when clicking the support email on the About screen.

Changes

• Fix adding secondary mailto: prefix to the URI before launching it

Checklist

• My code follows the style of this project.
• The code compiles without warnings.
• I have performed a self-review of the changes.
• I have documented my code, in particular the intent of the
  hard-to-understand areas.
• (If necessary) I have updated the CHANGELOG.