Adjust Branch Rules Settings UI to new designs

What does this MR do and why?

Adjust UI to new designs

• changing card title
• changing cards to new card color scheme
• improving loading state
• removing duplicate strings
• removing redundand methods
• adding missing alt text for avatars
• correcting headings hierarchy

See comments in the code for more details

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After
Rule_details_loading_state_before	Rule_details_loading_state_after

Note: This will temporarily show only No data to display for empty rules. This will be handled in !148412 (merged).

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Open Rails console. In your terminal: rails c
2. Enable the feature flag:

Feature.enable(:edit_branch_rules)

2. In a project, go to Settings / Repository
3. Expand Branch rules section
4. Click on View details of a chosen rule or use Add branch rule button if you don't have any yet.

Related to #388129 (closed)