Refresh: Popover tweaks

Created by: umpox

Addressing feedback from https://github.com/sourcegraph/sourcegraph/issues/19927#issuecomment-843174212

Changes:

• Updates global dropdown-menu and popover styles to use updated refresh box-shadow and border-radius values
• Fixes inner input padding to align with branch/tag/commit node to match designs
• Removes additional padding on 'Show more' button in popovers
• Updates close icon color to match designs
• Remove fade in/out transitions on popovers
• Ensures nodes for commits are the same height as nodes for branches/tags