Ability for volunteers to sign on/sign off duty

[laddhoffman]

When a volunteer reports for their shift, they should be able to sign in.
This will show their status as active in the system.
When they go off duty they should be able to sign off.

Active/inactive status could be used as a flag to help control messaging.
A volunteer coordinator should be able to see an overview of active volunteers.
A volunteer should be able to see a list of other active volunteers.