-  Use a new method by_milestone in
EpicsFinder to fetch milestone from params
and filter epics that contain issues with
given milestone
- Add milestone_title argument to EpicsResolver
- Add Finder and Resolver specs for filtering by
milestone cases

Update GraphQL schema

Allow filtering by project milestones

Update milestone_title argument description

- Update GraphQL schema

Use hierarchy params to determine milestones

- If include_ancestor_groups or
include_descendant_groups params are present these
groups and their projects will be included in
MilestoneFinder

Add specs for filter by milestones with hierarchy