After this change: "http://<server>/icons" will point to "src/icons"
folder and "http://<server>/images" points to the real "images" folder
in the relative path.
Specifying a height/width value might cut the displayed text or
obstruct centering the text in the button, so let it auto decide
instead.
Closes#11553.
This cleans up the global namespace by explicitly exporting shared values. All html and JavaScript files have been converted to use explicit exports except for client.js and mocha-init.js