Basic incident model

Generated with the command:

rails generate scaffold incident title:string description:text
severity:string status:string started_at:datetime detected_at:datetime
Remove delete functionality from incidents

Incidents will be closed when we're finished with them, but we don't
ever want to remove them from the database.
Remove status from incident form

This will default to 'open' for new incidents and updated elsewhere.
Add validations and specs for incident model
Redirect root path to incidents index page
Merge pull request #3 from intercom/elk/incident_model

Basic incident model