Improve highlighting of results in the SearchDropdown

[seanmturley]

Description

Currently the highlighting of the currently focused search result (using mouse or keyboard) doesn't extend the full width of the SearchDropdown. This is functional, but ugly.

Possible improvements

• Extend the highlighting the full width of the SearchDropdown
• If the above is not practical, add some padding so the subreddit icons aren't right at the edge of the highlighted area.
• Depending on the approach taken above, consider adding border-radius where appropriate.
• Consider adding a border-radius to the bottom of the SearchDropdown (only tangentially related to the issue).

[seanmturley]

This work could be grouped together with #135.