Add style changes to epics roadmap to allow for labels and additional text to be displayed on timeline items

What does this MR do and why?

Adds style changes (and some class changes to templates) to the epics->roadmap:

• Changes background to white so that additional labels/text can be added onto epics in future (see #383204)
• Changes flex settings of epics and timeline so that rows will expand to fit content

Screenshots or screen recordings

How to set up and validate locally

View Epics > Roadmap with some epics populated in a project

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.

frontend roadmaps