SSBC: Incorporate editor into actual "spec" stage shown from execution

Created by: courier-new

Currently the CreateOrEditPage and ExecutionPage are completely separate, with a read-only version of the batch spec editor incorporated into ExecutionPage and no path to go back and edit a batch spec from the execution. The purpose of this ticket is partly feature-based and partly tech debt/refactoring:

• Merge the two components and reconfigure routes
• Use actual editor on spec stage shown from execution
• Keep editor read-only while execution is happening
• Make it clearer execution is happening when you're on a stage other than the execution stage
• Provide button to cancel current execution and edit the spec