Set main menu width in pixels

This avoids styling issues if menu and page content have different font
sizes. 240px is enough to keep all the menu items in one line in most
languages (tested in en, de, es, pl, ru, zh, ja, ar, he).

Bug: 63313
Change-Id: Ie73eac27d5ca6bcdd47354b0bf62b6dd85876986
Merge "Set main menu width in pixels"