Improve edge connection interface

[alexsb]

Right now it's tricky to understand which attributes match up between nodes and edges. We need a way to preview how many connections exist between two tables. This is the current interface:

Approaches to address this

• scale edge width by number of matches
• for selected source and target attribute, highlight all the matching cells in blue, all the non-matching cells in red.
• for selected source and target attribute, show a label between the tables that shows how many fields match ( 133/181 rows match )
• for a selected source, show all possible matches in the target
• auto-connect source and target that are most connected