When creating a custom skin, we shouldn't have to explicitly set the baseskin

I found this bug while fixing XAPM-22.

• At this application, there was a custom skin created that contained a modified template that has nothing to do with Flamingo or Colibri skin.
• That skin was applied on Project Management application and the baseskin property didn't have a value. I guess that skin was created on an XWiki instance that had as the default skin - colibri skin - , because as you can see from the issue, the bug is found only on flamingo.
• Therefore, when the XWiki instance is upgraded (i.e. the default skin is probably changed), we have to set the baseskin property to the default skin of that certain instance to have a good display, which I don't think is user friendly. It should fallback on the default skin if we don't set a value to baseskin property.