Clean up group and project milestones

What does this MR do and why?

Cleans up group and project milestones

• Left aligns forms
• Unify spacing
• Remove unnecessary borders and wells
• Use Save changes instead of Update milestone on edit form

What does this MR not do?

• Removes title borders (Remove bottom border from page titles (!114017 - merged))
• Mobile align due date links (separate MR)
• Combine some header action buttons into a dropdown

Screenshots or screen recordings

Project milestones

Before	After

Group milestones

Before	After

How to set up and validate locally

• Create, edit, and view a project milestone
• Create, edit, and view a group milestone

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.