Refactor files action of FilesController

Introduce service object RepositoryFile which
contains the logic of file and directory
retrieval on model level.

Introduce GitFile which contains logic of
file and directory retrieval on git/lib level.

Those classes separate the concerns of git/lib
and ontohub models way more.

Use InheritedResources for FilesController
and simplify files action (will be renamed later)
split history action off of files controller
fix remaining merge error
fix syntax error
code style