Fixing disappearing hamburger menu for narrow screens (#3929)
* Made top search bar more scalable for (physically) narrow screens
Removing `flex: auto` on `.top-left-block` for screens narrower than
`350px`allows the search bar to resize to fill up the remaining space.
Setting `width: 70px` on `#typeahead-container input` for mobile widths
prevents the hamburger button from being squished at certain widths.
Given that the padding adds up to `50px`, setting what amounts to the
minimum width of the search field to `70px` seems reasonable.
Fixes #3199.
* Making hamburger button margin and spacing narrower for mobile
The clickable area is restricted to the icon anyway, and the extra space
benefits the search field width on narrow screens.