Configuration defaults repeated/scattered

[boyska]

Now we have some configuration options defined only in conf/defaults.py, some of them duplicated between code/defaults.py and LibreantCoreApp, some of them which are only stated in LibreantViewApp. See also ael-code#4

The point of this issue is to take a decision as in if/what should be duplicated, based on what criteria, and why, so that there can be coherence in the future. To close this issue both code change (if needed) and documentation (in any case) should be produced.