Add expiration_date and can_expire columns to the Resources table

[tammytee]

This will be a first step at making sure we're consistently auditing our resources for outdated content. Once a week, we should send a message to the ops channel in Slack with a compiled list of all of the expired resources.

Each resource should have a default expiration of 6 months from the date they were added. If can_expire is false, then an expiration date is not required for that resource.

Ensure that we are able to modify these fields via Nova

• expiration_date: date picker field if available in Nova
• can_expire: checkbox