Fixes #3310: Preselect cable fields

[hSaria]

Fixes: #3310

Preselect the site and rack fields when connecting a cable. Also noticed that the status, type, and length unit weren't using Select2, so fixed that one up as well.

Because of how chain filtering cannot span multiple model relationships (AFAIK), I cannot filter the list of interfaces based on the site and rack; this change only assists with the devices field.

[jeremystretch]

The root issue here was trying to set initial data in the form's __init__() after it had already been modified by the mixin. Ideally, initial data should be applied from within the view rather than from within the form itself. I've created PR #3987 to resolve this, which incorporates your other changes here. Thank you for your work on this!