Change 'name' field in Sources to 'Domain' (or better, 'cannonical domain') , add validator on ui

[pgulley]

Some sources, particularly those with url_search_strings, end up with descriptive names in the 'name' field instead of their canonical domain. This is mostly a case of poorly documented behavior- but we use the 'name' field to generate the 'canonical domain' filters when we query elasticsearch- so these sources are broken.
Changing the field name to something more descriptive, and adding a validator to make sure users are entering the right kind of data, will keep this from happening again.

I'll also spin up some more source health tickets to fix the cases where this has already happened.