Added Project Snippets to API.

Summary:
Added Class ProjectSnippets
Fixed comment
Corrected placeholders and fixed linting
Merge branch 'master' of https://github.com/KWARC/MathHub into GitLabApiWrapper

Test Plan: Tested by hand.

Reviewers: m.iancu

Reviewed By: m.iancu

Differential Revision: http://phab.code4fun.de/D92