Some cleanup and ODEO support

We've made some fixes to the SkillsFeed:

• author is not forced to join the lesson to have full access to it


• you can't rate your own lesson - we're sorry :)


• Slideshare changed its layout, which caused problems with our bookmarklet - it's fixed now


• duplicated URL and EMBED submission using bookmarklet is detected and reported as an error

Additionally, ODEO is now supported by SkillsFeed bookmarklet. Contributing podcast as a lesson is very easy now!