Update merge conflict page

What does this MR do and why?

• Add relevant breadcrumbs
• Keep Merge requests active in sidebar
• Remove border
• Add spacing between buttons
• Fix border radius on bottom of file
• Add . at end of sentence

Screenshots or screen recordings

Before	After

How to set up and validate locally

• Open an MR with a merge conflict
• Click Resolve conflicts in merge widget

#394879 (closed)

#396298 (closed)