Allows starting an already instantiated manager

When a manager is injected through other means, (e.g. using a different
GuiceModule and injector in Gerrit) we need a way to start it without
having to necessarily passing through the injection process again.
Allow different aspect rations for top-left logo

When configuring a different top-left logo, we would like to allow
to have a different HxW aspect ratio image. We need then to avoid
locking down the height of the logo and just keep the existing aspect
ration as-is.
Use startManager(X) from startManager(Injector, Class)
Merged #227 "Small adjustments to allow integration with Gerrit Code Review"