Add new status attribute to DoorCode, and remove obsolete door-code-related code

[anaulin]

This PR prepares the database schema for implementing the new door code workflows outlined in #594. In particular, this change:

• adds a new status attribute to a DoorCode
• makes it possible for a DoorCode to not be assigned to a user
• removes the enabled attribute from DoorCode, which makes no sense in the current workflow
• updates logic when removing key_member state to unassign the door code and set to the appropriate status
• removes the old door bell code (which relied on things like the enabled field, which is no longer available)

[compwron]

I took a quick look and it looks reasonable. Watch out for rails pre/post deploy migrations, you might need to restart the app after deploying since a column was removed

[anaulin]

Thanks for the review @compwron! I'll be sure to watch out for the migrations during/after the deploy.