Refactor endpoints

For naming consistency I propose the following new endpoint names for POST requests:

New Endpoint	Old Endpoint
/bg/add	/bg
/bg/remove	/clear
/bg/clear	/clear/all

The functionally of GET requests to /bg should be kept.

Rationale

All endpoints working on background knowledge work on prefix /bg. The suffixes /add, /remove, and /clear convey their meaning more concisely than the original paths. The GET request to /bg is clearly separated from other requests.