add method to find all sentences

The combined_sentences method will fetch all sentences, own
and imported ones. This also includes the imported_ontologies
method, which works as the name suggests.
add method to allow displaying all or subset-view

This includes the Helper (view and controller) to
determine if we want to see all (logics or sentences for example)
or only a subset.
allow sentence display mode switching.
mark every imported sentence with its origin.
add method to recursively fetch link-targets

This module utilizes a new layout model
for the graph-like structure-fetchers.
Later on (during *make fois version more stable*)
we should restructure the graph_data_fetcher into
this new layout.
use recursive fetch for imported ontologies.
use hierarchical structure for sentence locale.
add spec for fetching imported ontologies.
Merge branch 'staging' into 642-add_all_sentences_view

Conflicts:
	config/locales/en.yml
Merge pull request #675 from ontohub/642-add_all_sentences_view

642 add all sentences view