🥛 Make app navigation semi-transparent

[ChristophWurst]

As per the mockups

the app navigation component should become semi-transparent and let the background image show through with a blur.

Specification

• 80% opaque white with 10px blur, like in Dashboard. Code: background-color: var(--color-background-translucent); -webkit-backdrop-filter: var(--background-blur); backdrop-filter: var(--background-blur);
• AppNavigationNew gets our Nextcloud fade, looks nice and clicky
• WCAG AA: Contrast of secondary text in the navigation works after darkening it to 5c5c5c
• WCAG AAA: Just like for Dashboard, the high contrast theme should make the navigation background fully opaque and the secondary text color also color-main-text.