Add JSON Schema validation for AEG and Proof JSON

[James-Oswald]

JSON schema will allow us to ensure users only upload valid proof files and let them know what is wrong with their proof file if they upload an invalid one. If the compression issue #268 is done before this, compare to the schema only after uncompressing the content.

• Create a JSON schema for AEG JSON files as well as proof JSON files.
• Validate any uploaded proof or AEG JSON file against the schema.
• if it is invalid, display a visible error message where the error is in the json.
• See here for validator libraries.