Make Search feature more accessible

[tomsedory]

Make following changes to all html files so Search feature in header is more accessible:

• Fix accessibility error "Image button missing alternative text" by adding alt="" to Search images. #175

• Fix accessibility error "Missing form label" by adding aria-label="" to search images, search button and search field. #176

• Fix accessibility error "Orphaned form label" by adding id="search" to search button to tie label to button.

• [x] category.html

• [x] emergency.html

• [x] index.html

• [x] map.html

• [x] provider.html

• [x] providers.html

• [x] subcategory.html

• [x] terms-of-use.html