DatabaseInstaller had some LSP violations, such as calling undeclared
methods of the supplied Installer object, assuming that it was a
WebInstaller. It was also large.
So, split the web form parts of DatabaseInstaller into separate classes.
We have a class hierarchy for the connect forms, and a class hierarchy
for the settings forms, with a base class DatabaseForm mostly as a place
to put protected helper methods.
Also, have DatabaseInstaller::getConnection() return a special subclass
of Status, so that we can remove many Phan type overrides.
Change-Id: Ie84025f8f70b895fa6882848b9a21ba1750d60e2
