Fix two minor main menu bar CSS issues

Summary:
Ref T13263.

• Make the user profile section of the "Profile" dropdown menu have a transparent background, not a white background. This is a pre-existing issue. This is normally hard to see, but visible on Workboards with custom background colors.
• Fix an alignment issue with the little "V" caret in the search scope dropdown. This is a recent issue caused by some tab-caret CSS I added recently for tabbed dashboard panels.

Test Plan:
Before:

After:

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13263

Differential Revision: https://secure.phabricator.com/D20388