[IMPR] Derive FlowPage from ABC
[IMPR] Derive FlowPage from ABC

FlowPage is a base class for Board or Topic. FlowPage cannot be used
directly due to the missing _load method.

- derive FlowPage from ABC metaclass because it is a base class for
  Board and Topic
- add _load method to FlowPage and use abstractmethod decorator for it
- remove FlowPage._load_uuid() method and use FlowPage._load() in
  uuid property instead
- change position for parameters in private method _load for consistency

Change-Id: Ieab9c240f3ee757c7cf525c507909af88cd652b2

wikimedia/pywikibot-core,wikimedia/pywikibot-core