Description of the problem, including code/CLI snippet

Discussion objects seem totally missing from the python gitlab bindings. Discussions allow to link notes to a particular diff portion on a merge request, and link notes to other notes as replies, etc. Without discussions, most merge-requests notes are mostly useless, due to important info being missing.

Mapping this part of the API in python-gitlab would be very useful for apps about review for example.

Specifications

• python-gitlab version: 1.3.0