Signal Desktop: v1.15.0-beta.2 Release

Added a new option to disable spell checking for users who write in multiple languages that differ from their system locale.

Automatically clean up old debug information, remove logging unnecessary for troubleshooting purposes, and improve debug log submission.

Visual changes: - Conversation header: Ensure that profile names have tilde prefix - Media Gallery: fix handling of videos - show thumbnail and overlay - Dark theme: Brighten icons on image overlay for visibility - Lightbox: Play video on open, play/pause on click of video itself - Search box: Make entered text higher contrast - Left pane: Fix timestamp alignment on platforms with scrollbars