Implement group by milestone

[NereusWB922]

Summary:

Fixes #306

Type of change:

• ✨ New Feature/ Enhancement

Changes Made:

• Refactor Milestone model to implement Group interface
• Implement MilestoneGroupingStrategy
• Include Milestone grouping in GroupingContextService
• Add card view header template for Milestone grouping
• Add hidden groups card template for Milestone grouping

Proposed Commit Message:

Implement "Group by Milestone"

WATcher's default view displays issues and pull requests based solely on
their assignees. Grouping issues and pull requests based on their
associated milestones may provide another useful view for the user.

Let's implement the "Group by Milestone" feature.

[nknguyenhc]

LGTM! Clean implementation of the grouping context service and the milestone grouping strategy.