Search component breaks layout with increased font sizes

[andysellick]

If you increase the text size in your browser the search component input can expand below the height of the button, as shown (you can simulate this in Firefox with the 'zoom text only' option enabled, and zooming).

I had a quick go at fixing it and it might be possible to give the .gem-c-search__submit button element a position absolute and height of 100% to fix it, but I've not tested it in other browsers.

Noticed as part of #4218

[andysellick]

Definitely don't go with the fix I suggested above, because it does this with the new autocomplete...