Add REST endpoint to query & delete posts (messages)

[andreaskoepf]

In the DB we are dealing with message trees. We need REST endpoints to query single messages, conversations & message-trees.

post_id here means PK of post table in db. The frontend-provided (only in combination with api_client_id unique) id will be called frontend_post_id.

Required functionality

• query a message by post_id

• query a message by frontend_post_id

• query a conversation given a post_id (list of messages traced to tree-root in order (starting at root))

• query all messages of a message tree given a post_id (lookup thread_id of message -> query whole tree) (unordered)

• query children/descendants given a parent_post_id (with max_depth)

• query longest conversation of a message tree (by providing a post_id of any message in the tree)

• query messages of tree with most direct children (by providing a post_id of any message in the tree)

• query by user_id, ordered by date with conditions: max_count (limit), after_date (optional) & before end_date (optional)

• query by created_date, max_count (limit), start_date (optional), end_date (optional), only_roots (optional)

Only trusted api_clients:

• delete message (e.g. by setting deleted flag to true)
• delete all messages of a user (select all user's messages and perform delete them)
• query stats: number of total entries in post table, number of threads (message trees)

Optional (using existing functions):

• sample random messages
• sample random thread

[andreaskoepf]

change regarding delete: only flag messages as deleted, actual data removal has to be done by admins.

[andreaskoepf]

At API surface-level the new naming scheme should be used, see #124.