Add Celsius/Fahrenheit Dropdown converter to Fetch Block

[rsalsbery]

This case is being opened so that we can add an option to convert Celsius to Fahrenheit, and Fahrenheit to Celsius in the External Fetch block, as additional options in the "Number Manipulation" dropdown:

Additional options should be labeled "Convert to Celsius", and "Convert to Fahrenheit".

[Dan-Peck]

Merged in @ 69c76a0 and added to the queue for upcoming release to the live site.