Sticky Avatars for Mobile

Hello :wave: This theme component is activate sticky avatars for mobile.

I reused the core sticky avatars feature to make it useable on mobile.
The whole section (topic avatar + topic meta data) is sticky to top. If the post height is out of screen long or if scrolling up on long posts. I removed the sticky avatar for short posts when scrolling up so it will only activate on long posts on mobile.

I disabled this feature on personal messages with some css overwrite.

Because on the sticky section I set up a background color var(--secondary), I moved the topic post highlight from .topic-post to .topic-post .contents so only the section below avatar and meta data will highlighted. :arrow_down_small:

On this post only the contents section is highlighted.