GraphQL support

[dcoraboeuf]

Provide an end point to query entities, like projects, branches, builds, etc.

Using GraphQL.

• [x] GraphQL schema skeleton
• [x] GraphQL end point
• [x] GraphQL lists
• [x] GraphQL schema
• [x] In-app query (GraphiQL?)
• [x] GraphQL schema extensions
• [x] Documentation of core features
• [x] Documentation of extensions
• [x] Integration with DSL

[dcoraboeuf]

Consider using GraphQL. Would be a nice fit.