Unifying search box styling across Settings and Search pages

[MundiaNderi]

Fixes #4264

Unifies search box styling in the search page and settings page, so that it's similar to the search box on the resource page drop down and copy from locale batch action drop down.

To test:

• Check if the changes apply to all the search boxes.

[functionzz]

Hey @MundiaNderi, @mathjazz is on vacation, when he comes back he can take a look and approve the design - thanks!

[mathjazz]

Thanks!

What about other pages, e.g.:
https://pontoon.mozilla.org/sl/permissions/
https://pontoon.mozilla.org/messaging/
https://pontoon.mozilla.org/admin/projects/
...

Are those meant to be fixed in a separate PR?

[MundiaNderi]

Thanks!

What about other pages, e.g.: https://pontoon.mozilla.org/sl/permissions/ https://pontoon.mozilla.org/messaging/ https://pontoon.mozilla.org/admin/projects/ ...

Are those meant to be fixed in a separate PR?

I had missed those, thanks. No, I'd rather address them in this PR.