Update post statuses

[jasonmule]

This pull request makes the following changes:

• Add 'under_review' and 'archived' statuses to posts
• Remove 'draft' status

Test these changes by:

• Change post status in the client by choosing 'published', 'under_review' or 'archived'.

Fixes #1281 .

Ping @ushahidi/platform

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This change is

[coveralls]

Coverage increased (+0.004%) to 70.054% when pulling bbe3efa on 1281-change-post-statuses into 3af061e on develop.

[coveralls]

Coverage increased (+0.004%) to 70.054% when pulling 673f200 on 1281-change-post-statuses into 3af061e on develop.

[coveralls]

Coverage increased (+0.03%) to 70.08% when pulling b541442 on 1281-change-post-statuses into 3af061e on develop.

[rjmackay]

Seems perfectly good to me. Just need to update tests.

[jasonmule]

@rjmackay I actually ended up using draft to track Under review