Add verifiability and priority information to oncall_contacts

Closes #12 (closed)

Closes #18 (closed)

• Added priority and verifiable fields to the oncall_contacts table
• verifiable is boolean value that is automatically filled out by Hot Potato, and is determined by what sending method the on-call contact is: True for app and Pushover, False for pager and SMS
• priority is an integer value where 0 is the highest priority
• Added the ability for the user to assign a priority to each on-call contact method
• On-call contacts are distinguished by their verifiability in the UI
• The "Add contact method" page no longer crashes when no details have been supplied, it now prints out helpful messages to the user, and retains all information input by the user so far
• The notification sending queue has been reworked to send by on-call contact rather than sending method. It sorts this iteration according to verifiability, then priority
• Generated a migration to retroactively add the apprioriate priority and verifiable values to all existing on-call methods. Lots of reworking of the migration system went into this to make it possible. Unfortunately, this involved separating the schema and data migrations into two separate transactions, since CockroachDB does not appear to support accessing table changes mid-transaction at this time
• Fixes the robot-test Docker Compose service by making the image URL all lowercase