refactor proof for user view

* proof for user view moved to requirements#index
* replace create with build in `proofs_controller` to stop writing to
database twice
* change redirect page to `requirements_url` after sign in
* move marketing copy from requirements#index to home#index
* add navigation link to go to requirements#index for user
* fix homepage integration test
Merge pull request #30 from akshatpradhan/move-requirements-view

refactor proof for user view