Migration Script: Convert app.redash.io csvurl format queries to csv/excel format

[susodapop]

Description

The primary users of the forthcoming redash-migrate command are users of Hosted Redash, which includes a data source called csvurl which is not part of the open source version. redash-migrate ignores these data sources and their queries as a result. But we should add a mapper that creates a CSV type data source. It should also convert the queries like so:

-- Plain text format used by csvurl

https://www.website.com/path/to/file.csv

-- YAML format used by CSV

url: "https://www.website.com/path/to/file.csv