add post creation via the API

[Xe]

Before you begin

• I understand my contributions may be rejected for any reason
• I understand my contributions are for the benefit of the Philomena software
• I understand my contributions are licensed under the GNU AGPLv3

• I understand all of the above

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This offers the following routes (given as a RFC 6570 URL template):

route	why	needs
POST /api/v1/json/forums/{forum_slug}/topics/{topic_slug}/posts	Creates a new post in a forum by slug's thread by slug	?key={your_api_key} and {"post": {"body": "textile markup", "anonymous": true/false}} as the body

This also announces new forum posts to the Firehose API like they would be if they were made using the web UI.