Rework Dataset File Schema creation functionality

[jamesjefferies]

Given more time the Dataset File Schema creation functionality would be refactored and tidied up. There are a number of areas where it is messy namely:

1. Sometimes the DatasetFileSchemaCreation service is used, sometimes it isn't
2. Schemas get parsed and validated using both CSVLint and the JSON Table Schema gem. There is some tidying up do be done there
3. The parsed schema is stored in the model as JSON, but isn't always used for validation, as for example, CSVLint wants a URL to parse rather than a string, whereas JSON Table Schema handles any IO object.
4. The interaction with inferred schemas isn't always as clean as it should be
5. The forms use url_in_s3 as a reference, whereas actually this field could be a reference to an uploaded file, such as when Javascript is not switched on. It would help for this to be consolidated across all file uploads to be file_reference or similar.