Update about.html so Search is more accessible

[tomsedory]

Update about.html so Search feature in header, visible in desktop and mobile sizes, is more accessible. Made following changes:

• [x] Fix 2 accessibility errors #175 "Image button missing alternative text" by adding alt="" to Search images. #
• [x] Fix 1 accessibility error #176 "Missing form label" by adding aria-label="" to search images, search button and search field.
• [x] Fix 1 accessibility error "Orphaned form label" by adding id="search" to search button to tie label to button.
• [x] Fix intrusive alert for accessible technology by removing <p id="error" role="alert"></p>.