Give LoncapaProblem its own LoncapaSystem instead of ModuleSystem

LoncapaProblem was using ModuleSystem directly, but we're about to need
i18n passed in, so this gives LoncapaProblem its own LoncapaSystem
object to better module its dependencies.  This commit simply passes
through the attributes of ModuleSystem that Loncapa needs.

[LMS-1597]
Add i18n attribute to LoncapaSystem

[LMS-1597]
Use self.runtime instead of self.system

[LMS-1597]