Unify search box styling across Locale, Project, Resource menus and the rest of the app

[MundiaNderi]

Follow up from #4152

Right now there are two different visual styles for search inputs:

• In the translate view, the search boxes in the Locale, Project, and Resource menus look like input fields with a full border.
• Elsewhere in the app, search boxes only have a border at the bottom.

We should unify these so search box styling is consistent across the app, rather than having two divergent patterns depending on context.