Add a way to export user content via the REST API

[oplik0]

Description

The read API contains three export endpoints (/api/user/{userslug}/export/posts, /api/user/{userslug}/export/uploads and /api/user/{userslug}/export/profile) that for anyone actually wanting to get these files programatically will appear not to work.
The reason is that they don't actually try exporting anything, instead only letting one download a pre-existing export, which is done via users rights and consent page, or socket.io (there doesn't appear to be anything in write API related to this functionality).

So it'd be good to either have these endpoints try generating the export if it doesn't exist yet, or have new endpoint(s) for that purpose.

EDIT: this actually worked in the past. It's a regression back from v1.14, caused by this commit 8383992

Community forum reference

https://community.nodebb.org/topic/16615/export-user-s-posts-in-csv

[julianlam]

Yeah makes sense. A new set of API routes would be handy.

• POST routes to generate a new export
• Restrict generations to one a day unless admin Locks already exist in existing export generation code
• HEAD to return Last-Modified or HTTP 204 if no export generated
• GET sends the file, or HTTP 204 if no export generated
• Deprecate socket.io calls in favour of api v3 calls
• Breaking In a separate PR, remove the old routes

Likely will follow the existing schema: /api/v3/users/{uid}/export/posts, and so on...