isModuleInstalled added to BackendModel
Changed name to isInstalledModule()

Conform functions like BackendAuthentication::isAllowedModule() and
isAllowedAction().
Removed BackendExtensionsModel::isModuleInstalled()
Replaced BackendExtensionsModel::isModuleInstalled() by BackendModel::isInstalledModule()
renamed isInstalledModule to isModuleInstalled
fixed login sql error on wrong email
Email: allow null as plain_text value.

Fixes #429.
Merge pull request #421 from gertjanmeire/login-bugfix

fixed login sql error on wrong email
Resolve conflicts.