Add "view" projects API

[towerofnix]

That is, the endpoint that registers that you have viewed the project (hence increasing the view count), /users/${username}/projects/${id}/views. Reference code in LLK/scratch-www#2417.

Per details in https://github.com/LLK/scratch-www/pull/2417#discussion_r239690551: username above is of the author of the project, id is the project ID. It does take credentials (auth token).

Looks like it might take CSRF token too, but that'll require manual testing to be sure.