Forums: Make search input wider

[SergeyBiryukov]

English and Rosetta support forums currently use different search forms, see [3926].

Search inputs in both forms have a couple of issues:

• The default 140px width is too short.
• The input on ​https://wordpress.org/support/ has an extra header and is missing bottom margin.

See the screenshots. The inputs also have different background color, but I'm not sure which one is "canonical", so the patch only handles the two points above.

[coffee2code]

In 4209:

Forums Theme: Fix display of search form on forum front page.

• Double width of input field
• Add margin below input field
• Remove unnecessary search form header

Props SergeyBiryukov.
Fixes #2120.