add uri fetcher to fetch corresponding files

we can either fetch them to memory or directly
to the harddrive.
add check if an ontology is imported
make user_info a function
add ability to delete a file in a repository
add destroy route to an ontology
add controller actions to delete an ontology
add button to delete an ontology to view

tooltip is not shown yet because the according bootstrap javascript is not initialized
fix source target error
fix tooltip on disabled delete button
should fix #612
small refactoring
extract_class_names is obsolete

hierarchical_class_names should be used instead. one test still fails.
related to #249
should fix new error in #249
add spec for ontology deletion
add OntologyDeleteError
improve tests on deleting an ontology
implement correct ontology deletion
Merge branch 'staging' into 398-delete_ontology_button

Conflicts:
	app/models/ontology.rb
	spec/models/ontology_spec.rb
fix syntax error in test
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