Expose a /release endpoint

[vincenzobaz]

Some Scastie bugs related to library dependencies arise from the need to query the releases available for a given project.

• Library lookup allows choosing a non-existent version of the dependency scastie#471
• wrong version when adding a library scastie#392

The ideal way to solve these issues would be to query a /release endpoint exposing the required information

[adpi2]

For a particular release we should return all available information:

groupId: String,
artifactId: String, 
name: String, 
version: String,
platform: String,
scaladoc: String,
dependencies: List[Dependency]

[adpi2]

This is addressed by #1345