Create enum for source field

[jmartinm]

The source.yml is currently a free text field. This will be a problem when we start using this field value as the way to retrieve records from different sources (arXiv, APS, ...) by the merger, since this value will be in the database and should always be the same.

An enum should be created instead.

NOTE: One of the values in the enum should be used for the user submission forms. The forms currently don't populate the source field, but they should start doing that once we have the merger working.

[jmartinm]

CC @michamos @ammirate