[Backend] Comments/answers API

[ege-kaya]

The comments and answers API has been implemented and works as documented on the API documentation in the wiki. The model classes were taken from the work of @ibrahimbayat and @supiket.

@KeremZaman, please keep in mind during deployment that this implementation requires a new dependency, version 0.7.1 of the drf-extensions package.

This PR will resolve #206 if merged.

[supiket]

Will review urls.py, views.py and tests.py for event posts and equipment posts at a later time.

[ibrahimbayat]

I think it is good. Implementation is little different from my implementation. Instead of getting comments from post id, creating two different modules and getting comments from equipmentpostid and eventpostid is more clear and easy way.

[supiket]

Smooth wrapping up of comments & answers, thank you for your work!

[KeremZaman]

Looks quite good! Renaming routes for equipments and events is also good in terms of clearness. Thanks for your tremendous effort to make it happen in such a short duration!