Remove redundant scripts

[kronosapiens]

Big clean-up of the codebase. This PR:

• Deletes the entire migrations/ directory
• Deletes a handful of unused scripts from the scripts/ directory
• Deletes the task-related test files and scripts
• Consolidates all test files into the tests/ directory
• Consolidates the .solcover config files into the .solcover/ directory
• Reorganizes package.json and .circleci/config.yml

[area]

As discussed on a call, need to check that we're not nixing anything that the CDApp dev environment uses.

[kronosapiens]

@area I've checked the deleted scripts and none of them show up in the CDapp repo

[area]

(Skipped) tests removed here can be pulled back in #717 if/when that is required.