Add CSS classes to search page elements

Problem/Motivation

Currently such search page elements as result count or help text cannot be styled because they don't have any css classes.

Steps to reproduce

N/A.

Proposed resolution

Edit hardcoded html markups in SearchApiPageController.php file.

Remaining tasks

Review the patch added.

User interface changes

Search page UI elements style can be customized.

API changes

None,

Data model changes

None.