Change default Wiki "First page name" to be blank

When creating a wiki the default value for "First page name" is "First page name". It should be blank and force the user to enter in a page name value, because if someone is not paying attention they will create a new wiki with a useless first page name and cannot change it.

The request to change the first page name is in MDL-6488, but until that is implemented, it is better to prevent someone from ending up with a horrible default name.