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 proof_statuses table in the databse represents the SZS ontology.
This table is populated in a migration and is not subject to be changed
by a user - just as Ontology.FILE_EXTENSTIONS.

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.
Remove unused SZS fixture file.
Merge branch '1114-add_theorem_support' into develop