JUcmContent can't construct from alias

Fixing the constructor for JUcmContent. If the $table isn't passed, the constructore is supposed to fetch the table using the $alias. This didn't work because the data isn't available where the constructore was looking for it.

Also fixing two issues with the install SQL files:
- The table value for #__content_types wasn't proper JSON. There is no such thing as "associative array" in JSON, it's an object then and has to be written with curly brackets.
- The weblinks entry for this table value was missing a quote.