Feature: Change the way we validate posts

[rjmackay]

Depends on #1283

Current validation

• Post fields are only validated the same other tasks.
• Only complete stages are validated
• Only enforces validation when post status = published

ie. validation is a combination of:

• Is this post published?
• Is this stage completed?
• Is this field valid?

Changes

• Required fields on "post" stage are always validated
• Required fields on tasks are only validated when it is complete?
• Update field validation logic in client: Always validate post fields, only validate task fields when completed. Post status has no impact on validation.
• API: Update post validator
• API: post status should have no impact on validation

Background:
https://github.com/ushahidi/platform-client/issues/147
#1188