Add/replace logo and cleanup overall branding

This MR cleans up logo icons for different platforms, replaces the logo icon with a new version and adds a logo component for the start page. Furthermore, the app description has been updated. These changes only effect the open source "arsnova" branding.

Today, all browers which support the configuration of additional logo icons support also support the standardized Web Application Manifest. Therefore, there is no longer a need to have browser-specific meta tags and config files. We do not support IE or old versions of browsers in general so there is no need to keep those configs for compatibility reasons.

• Existing icons have been renamed to browser-neutral names.
• Icons in uncommon sizes have been removed.
• Non-standard icon-related meta tags have been removed.
• Splashscreen images for iOS have been removed. They were never updated for new device generations anyway.

Apple HIG:

You can let the system automatically scale down your large app icon to produce all other sizes, or — if you want to customize the appearance of the icon at specific sizes — you can supply multiple versions.

See: https://caniuse.com/mdn-html_manifest_icons
See: https://developer.chrome.com/docs/lighthouse/pwa/installable-manifest/
See: https://developer.apple.com/design/human-interface-guidelines/app-icons
See: https://developer.mozilla.org/en-US/docs/Web/Manifest