Allow setting the source mini directory default value.

Currently, inside of Neos, you are able to add, modify, and remove entries from the Source/"How did you hear about us?" mini directory; however, if a user were to remove the "Other" as an option by disabling, or deleting this option it will continue to show up as it is the hardcoded default for that specific mini directory.

It would be best to allow firms/users to set the default Source mini directory, so that they may have their most used type of referral to be set automatically when making a case.