Add alt text to spacer elements

Add alt text to spacer elements

All HTML img elements should contain an alt attribute.
For decorative or spacer elements this alt text should be a blank string,
which indicates to assistive technologies that the image should not be announced.

Longer-term these spacer gifs could be replaced with empty <span> elements.
Inline-block css has been added to allow for this change in future,
however, the HTML element has not been changed for now to avoid breakage with non-default themes
(e.g. civicrm-admin-utilities WordPress plugin which replaces the default css/civicrm.css file)