[7.x] Add dynamic 'column' key for foreign constraints

[reed-jones]

This change enables using the new (7.x) foreign key ($table->foreignId('user_id')->constrained();) syntax on tables not using 'id' as their primaryKey. Previously only the table could be specified.

As an example

    $table->foreignId('festival_year')->constrained('festivals', 'year');

would be the equivalent to the previous syntax of

    $table->unsignedBigInteger('festival_year');
    $table->foreign('festival_year')->references('year')->on('festivals');

[tomstreeter]

I’ve complained in the past about the frequency of updates. I’ll stop now because this cleanly solves a problem I didn’t realize I had until yesterday.