Add new design for fuzzy finder

Created by: olafurpg

Last remaining blocking item for #21201 (closed)

Previously, the fuzzy finder UI used basic HTML/CSS that was inconsistent with the rest of the Sourcegraph UI. This PR implements the Figma design proposal in https://www.figma.com/file/ZYu2iiTp4AmnHDXf9EX2sY/File-fuzzy-find?node-id=0%3A1&viewport=2392%2C586%2C1 so that the UI now looks like this