Milestone Custom Fields Proposal

Here is a proposal for Custom Fields — based on the initial concept of Subtickets. See related tickets #31, #886 and #6328. The idea behind this, is in fact the opposite of the SubTicket concept. We already have the ability to add custom fields to tickets, so why not make Milestones have the same options.

Examples

For instance, I want to have a workorder number and client attached to each milestone. And I want that visible on the roadmaps (Can do manually of course with templates). Currently, I have to make another table that matches up to each milestone, create an entire plugin for the whole damn thing, and finally connect it. Tickets have this exact functionality already, Milestones need it too.