Feature: Distinguish between post fields and task fields

[rjmackay]

Changes

• API: Mark "Post" stage as special (maybe type: post vs type: task)
  • Post stage doesn't have complete/not completed
• API: Add description and visibility for stages
• Split post fields and task fields in survey settings
• Split post fields and task fields in post edit
• Split post fields and task field in post detail
• Remove 'completed' toggle from "Post"
• API: Add is_public field to post stages
• Add "Show this task to everyone when the post is published" to task settings
• Out of scope: Add display logic for "Show this task to everyone when the post is published"
• Out of scope: "ASSIGN THIS TASK TO..." dropdown

Background:
https://github.com/ushahidi/platform-client/issues/149#issuecomment-228809927
#1262 (comment)
#1263 (comment)
#1237 (comment)
#1237 (comment)
https://github.com/ushahidi/platform-client/issues/147

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