Create interface files compatible with solc 0.5 and 0.6

[sohkai]

As solc gets updated, users are increasingly frustrated that they are not able to build upon and integrate with aragonOS contracts which are pinned to 0.4.24.

Instead of updating the contracts themselves to be compilable under 0.5+, we can ease integration pains by providing solc 0.5+ compliant libraries for important contracts (e.g. various base and factory contracts).