The sorting method does not need to be stored in the backend db

[skydread1]

Problem

Sorting the posts using the creation/last-edition dates should be a frontend feature only.

There is no need to store a fixed method as the user should decide which ordering he prefers.

However, a default ordering is being worked on in #177 that would allow the user to specify the default order for his post.

Suggestion

• remove all unnecessary backend logic for the sorting
• sort the post by creation date by default in the frontend as for now
• remove sorting validation and pull pattern obsolete logic