WIP: Resolve "GitLab Flavored Markdown: colors"

What does this MR do?

This MR adds the ability to write text with any background color, with a {color} format, like so:

- {#f00} 
- {#008000}
- {rgb(255, 255, 0)}

The text will show with a box with the corresponding color next to it.

Are there points in the code the reviewer needs to double check?

• Added CSS file: app/assets/stylesheets/framework/color.scss
• Added ColorFilter in BroadcastMessagePipeline, SingleLinePipeline, GfmPipeline

Why was this MR needed?

This was a feature request, so colors could be added in Markdown text.

Screenshots

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• [ X] Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #24167 (closed)