Allow merge details section to be collapsed

What does this MR do and why?

Makes the merge details section in the merge request widget collapsible. This section can only be collapsed when on a screen size of less than 768px and it won't be possible to collapse it at all when in the ready to merge state.

Screenshots or screen recordings

How to set up and validate locally

• Open a merge request that has a blocked merge state (ie. unresolved discussions)
• Resize window to be 'mobile' size (anything less than 768px
• See collapse button

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.