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
fix MethodMissing error
outsource helper methods into HistoryHelper
rename HistoryEntries to HistoryEntriesPage
inherit from FakeRecord
documentation for HistoryEntriesPage
put attr_reader to the rop of the class
make compute_ref an instance method
un-protect methods that can't break anything
remove duplicate method
move attr_accessor to the top of the class
code style: remove empty lines
Merge branch 'staging' into 865-introduce_histories_controller
Merge pull request #981 from ontohub/865-introduce_histories_controller

865 introduce histories controller