Contend with time precision changes in Janus 0.9

[tgeoghegan]

The Janus 0.9 series will change how times are represented so that absolute times and durations are now multiples of a task's time precision. This has implications for divviup-api:

• will need to deal with both Janus 0.7 and 0.9 and up, based on an aggregator feature flag
• we need to decide how to represent this in divviup-api's API and the console
• some basic UX for this? Perhaps if we detect the TimePrecision feature, we put big red warnings on the task creation form saying "HEY DID YOU DOUBLE CHECK YOUR TIME MATH?"

[divergentdave]

We should differentiate between the different versions through the protocol field of the config struct, instead of a feature. This matches how we handled draft-04 in parallel previously.