search redesign: search box QA

Created by: limitedmage

Fixes #21318 (closed)

• search icon is too tall and icon should be centered
• adjust top margin for search button and toggles in xs size
• xs search box should have border radius
• adjust search box bottom border when under navbar
• add box shadow to search box
• equalize search button and version context dropdown height
• align xs search input with other elements

Not implemented:

• multiline, to be done in separately in #21215 (closed)
• adding placeholder text, Monaco does not support this (see https://github.com/microsoft/monaco-editor/issues/1228) but we can hack around it. to be done separately in #21465 (closed)
• modifying the autocomplete box position, this is handled automatically by Monaco and it would be quite expensive to hack around this.