Remove dependency on Panels

In preparation for upgrading farmOS to Drupal 8 (#2361735: [META] Map out Drupal 8 / farmOS 8.x-2.x Dependencies), we should reduce any module dependencies that are not strictly necessary.

Currently, the Farm Admin module provides the Farm Dashboard using Panels and Page Manager. The same thing can be accomplished with a simple page callback function that builds the layout we want. This would remove Panels and Page Manager as dependencies and therefore simplify the upgrade to D8.

Once we're on D8, we can reconsider changing how the dashboard is built so that it's easier to customize.