Merge request file nav file name and file path

Current implementation:

Design

• The width of the dropdown list is changed to 560px, so it can show the longer file path.
• The file name and file path are on different lines.

• First line --> File name
• Second line --> File path

• Ellipsis will be at the beginning of the file path if it's too long to show the full one.
• New icons for Additions, Deletions, and Changed files can be found on http://gitlab-org.gitlab.io/gitlab-svgs/

• file-addition
• file-deletion
• file-modified