-Alt text to image was set to hash, which would make the page display hashes if the image/gravatar  wasn't available, set the alt text to '', and made the main tool bar image follow the same or similar constraints as the other images (.avatar, .s26)

Conflicts:
	app/assets/stylesheets/themes/ui_bb.scss
	app/views/snippets/_snippet.html.haml

Change-Id: I42e1e86f535486fd2556d4b3e4591f3676eea033

Fix for https://github.com/gitlabhq/gitlabhq/issues/4157