Sort epics by start date and end date in the roadmap view and epics list view

What does this MR do?

Backend

It adds new sorting possibilities for epics, specifically sorting by end_date and start_date. The new sortings were added to the Epic model

It sets a new cookie in the EpicsController with key epic_sort where the last sorted key is stored. The same key is used in RoadmapController - these 2 sites share stored sorting between each other.

A new EE helper EE::SortingHelper was created and prepends SortingHelper. The new sortings, that are available only for epics were added there

Frontend

The new options - Start date and End date were added to the epics list and roadmap.

What are the relevant issue numbers?

#6494 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?

Closes #6494 (closed)