Refactor Post creation

[ralsina]

Some of the things that need fixing:

• Too many arguments for post creation (Fixed, now it gets access to config)
• All the searching for different language versions of the post should be done by the Post class
• Allow stories not to have dates (done in the new_galleries branch)
• Too many ways to specify metadata, with too many fallbacks
• Most of the posts.py task plugin logic needs to move inside the Post class (blocker for gallery refactoring)

[ralsina]

Dropping the last bit, will do that as a separate thing some day.