Employ MilestonesFinder in the Milestones API

Description

Employ MilestonesFinder in the Milestones API

Proposal

Make use of the MilestonesFinder to reduce code in the Milestones API and helpers

Expand the capabilities of the MilestonesFinder

Links / references

https://gitlab.com/gitlab-org/gitlab-ce/blob/3b3f0fab452fb9cbe4fbe7f75112bef3a7f9d039/lib/api/milestones.rb#L40-41