Allow for re-entry/updating of mturk keys #50
Description
Right now, once keys are entered, all tasks are lost if an update is needed. Instead, allow people to update their keys and maintain the experiments they have already added. (a simple button the re-prompts with the starting modal would work. then you can re-asssociate all current experiments / settings with the new keys).
Assume for now that people have to first sign in with the original keys they used, then update. This hopefully make it possible to avoid most issues with the hashing that is currently done on the server side.