use obo-label-names in sentences as well

This is solved by just calling set_display_text! on the
sentences at the right time (after the display_name for
the symbol has been updated).
add migration to update display_text of obo

Add the migration the update the display_text
of already existing obo sentences.
set displaytext in sentence only if non-nil.
optimize set_display_text! body.
improve conciseness of the obo-text migration.
Remove unused columns from sentences.
Add first draft of Theorem and ProofStatus models.
Exclude Theorem from Sentence scope.
Add Theorem route, controller and view.
Add ProofStatus model, remove STATUSES Constant.

The ProofAttempt and Theorem models now rely on this new model.
Add colored label to ProofStatus.
Add Popover for ProofStatus.
Parse Theorems.
Add specs on associations.
Add theorem parsing spec.
Use SZS ontology to create ProofStatus.
Fix seeds bug that deleted the proof_status.owl.
Create basic proof statuses for the tests.
Obey hound

In order to make the style consistent throughout each touched code file,
I also changed some lines that hound would complain about that were not
changed in this feature-branch.
move persisted? method to fakerecord