Roadmap milestones settings

What does this MR do and why?

Introduces the ability to turn off Milestones in sidebar and refine which milestones should be displayed.

This setting is not currently persisted in user preferences, only the URL params allow to persist it at this stage.

Behind :roadmap_settings feature flag.

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 the Roadmap settings title shows up
• Test Milestones radio buttons and toggle
• URL gets updated when changing options and milestones are updated.

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 #352331 (closed)