Introduce Roadmap settings sidebar

What does this MR do and why?

This is the first iteration of moving roadmap settings to a sidebar. Full context and designs in the issue: #345158 (closed)

This MR adds a settings button and an empty drawer for roadmap settings sidebar behind a new feature flag roadmap_settings

Screenshots or screen recordings

How to set up and validate locally

• Turn on feature flag :roadmap_settings
• Visit a non empty roadmap
• Click on Settings in the top right corner
• A drawer with just the Roadmap settings title shows up
• To close, click the button again or the cross in the drawer.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #345158 (closed)