Fix mobile menu after adding help link
After https://github.com/gitlabhq/gitlabhq/pull/6897 the mobile menu ran off the left side of the screen. I don't see much we can do to squeeze all the items into mobile's width. I think hiding one of the items is acceptable and which is a matter of opinion. I suggest hiding the user's avatar on mobile but the search icon or help icon would both be options as well. Let me know if you want a different icon hidden on mobile.