Moderation: new table to store price & proof reports

[raphodn]

Story

We'll be using our own moderation mechanism.
To do this we need a way to store reports from users.

What

A moderation table with

• the price or proof id
• a type (with some defaults + "other")
• a comment
• a status
• owner
• creation & modification date

And as we get more comments, we'll start building a list of types

Todo

• new Flag model (in the moderation app)
• new endpoints to allow adding flags on a given price or proof
• new endpoint to allow moderators to visualize all the flags + update them (change the status)
• API documentation: group moderation endpoints together + 1 sentence explanation for each