Fix hover state of Wikia Poll placeholder in RTE

Basically hovering on the placeholder was broken as described in [FB#101586](https://wikia.fogbugz.com/default.asp?101586). The cause of that was missing width attribute for img tag. It was used in RTE overlay plugin to calculate position of "modify remove" element (method getImageWidth() in /extensions/wikia/RTE/js/plugins/overlay/plugin.js). I removed the size from *.scss file and add the attributes (width and height) to the placeholder. Also, I found a piece of code in RTE.class.php which wasn't executed. @wladekb @macbre @owend you guys probably know RTE quite well -- can you take a look at this pull request and merge it if it's fine? :)