CSS/JS changes

Few bug fixes and enhancements.
1. Fixes https://github.com/mybb/mybb/issues/737 completely.
2. Fixes https://github.com/mybb/mybb/issues/976
3. Moves cookie plugin from general.js to jquery.plugins.js and jquery.plugins.min.js
4. http://community.mybb.com/thread-156278.html
5. Fixes typos/forgotten code introduced in earlier PR (image scaling).