Align plugin-manager header bar with Gerrit

Use the same 'Roboto' font as the Google screen and align the style of
the top-level bar, including the search box. Also, swap the action
button font with the 'Open Sans' used in the Gerrit header.

In order to reuse the same Gerrit fonts, import the /styles/fonts.css
from the plugin-manager and intro pages.

Old custom UI            : https://www.pasteboard.co/_EDQs3sdyOEU.png
UI more aligned to Gerrit: https://www.pasteboard.co/pUNu1B9guRTP.png

Release-Notes: Align plugin-manager and intro headers with Gerrit L&F
Change-Id: I8248c2e67e29033ffde779eade6aeeb7684d6b75