Workflow assignment messaging

[srallen]

Gravity Spy has a hardcoded message for when a volunteer is assigned a new workflow:

Right now, this is Gravity Spy specific and is the same message between each 'level'. I'm less certain of the utility to allow projects to customize this message versus us coming up with good generic language and I think specifics on the workflows should rather go into the tutorials and task help. It's possible there may be a good reason I'm not thinking of to allow customization of the message here, though, so I'm opening this issue as as placeholder for this. If so, I'm unsure if we should support a single message (maybe on the project resource?) or a message per workflow.