Add a "notes" field in the license manager#248

This is mostly relevant for Kirby 2 licenses as there is just a long list of of “Kirby 2 Professional License” at the moment.

Things to put in there:

• Which project/domain a license belongs to
• what the actual license key is
• if the site is still active or the license could maybe be recycled
• whatever else is relevant to that specific license.