Add support to distinguish between different access level

There is currently only two "levels" in ghostflow.

• Contributor -> everyone get this level, even non members of the project.
• Developer -> Onle "Developer" in the project get this level.

It would be nice to have three levels:

• Extern -> non members of the project
• Contributor -> member of the project < Developper
• Developer -> member of the project > Developper

This would give more rafinement to which level have access to which actions, eg:

• Extern: Reformat
• Contributor: Reformat, Test
• Developer: Reformat, Test, Merge

Related to #88 (closed)