Web installer fixes (issue #18142)

Fixes for http://issues.bestpractical.com/Ticket/Display.html?id=18142

Installer currently carks with undefined value warnings because $RT::Logger
is not initialised until most of the Installer is complete. Added force
initialisation of Logger in rt-server.

Need to actually execute the prepared statement to check for existence
of schema in Install/DatabaseDetails.html.